System for performing computer-based operations

ABSTRACT

A system for performing computer-based operations is disclosed. The system includes a printer for printing visible information and coded data tags on a surface substantially simultaneously. Each coded data tag encodes a region identifier for uniquely identifying a description of a region on the surface, and a position in the region. The description associates at least one instruction with one or more positions in the region. The system further includes a sensor for sensing at least one coded data tag and for generating a signal indicative of the sensed coded data tag. A computer system receives the signal, determines the region identifier and the position, relates the position to the at least one instruction using the description, and executes the at least one instruction.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. application Ser. No.10/971,146 filed on Oct. 25, 2004, now issued U.S. Pat. No. 7,469,830,which is a Continuation of U.S. application Ser. No. 09/575,188 filed onMay 23, 2000, now issued U.S. Pat. No. 6,832,717.

FIELD OF INVENTION

The present invention relates to an apparatus for performingcomputer-based operations via a networked computer.

The invention has been developed primarily to provide a surface-basedinterface which allows a user to interact with networked information andto obtain interactive printed matter on demand via high-speed networkedcolor printers. Although the invention will largely be described hereinwith reference to this use, it will be appreciated that the invention isnot limited to use in this field.

CO-PENDING APPLICATIONS

Various methods, systems and apparatus relating to the present inventionare disclosed in the following co-pending applications filed by theapplicant or assignee of the present invention simultaneously with:

6,428,133 6,526,658 6,315,399 6,338,548 6,540,319 6,328,431 6,328,4256,991,320 6,383,833 6,464,332 6,439,693 6,390,591 7,018,016 6,328,41709/575,197 7,079,712 6,825,945 7,330,974 6,813,039 6,987,506 7,038,7976,980,318 6,816,274 7,102,772 7,350,236 6,681,045 6,728,000 7,173,7227,088,459 09/575,181 7,068,382 7,062,651 6,789,194 6,789,191 6,644,6426,502,614 6,622,999 6,669,385 6,549,935 6,987,573 6,727,996 6,591,8846,439,706 6,760,119 7,295,332 6,290,349 6,428,155 6,785,016 6,870,9666,822,639 6,737,591 7,055,739 7,233,320 6,830,196 6,832,717 6,957,76809/575,172 7,170,499 7,106,888 7,123,239 6,409,323 6,281,912 6,604,8106,318,920 6,488,422 6,795,215 7,154,638 6,859,289

The disclosures of these co-pending applications are incorporated hereinby cross-reference.

BACKGROUND

Presently, a user wishing to interact with a computer system usuallydoes so using a display device such as a computer monitor and an inputdevice such as a keyboard or a computer mouse. Whilst such interfacesare useful, they are relatively bulky and non-portable, and can lack theimmediacy and expressiveness afforded by a pen applied to paper.Moreover, in many situations, paper is a more comfortable and convenientrecording medium, due to its superior visual characteristics and itsportability. Nevertheless, information recorded on paper is generallynot as accessible and malleable as information recorded on a computersystem. Furthermore, unlike a computer interface, a pen applied to papergenerally lacks the ability to control computer software.

OBJECT

It is an object of the present invention to combine advantages ofpaper-based recordal of information and computer interfacing.

SUMMARY OF INVENTION

According to an aspect of the invention, there is provided a system forperforming computer-based operations, the system comprising:

a printer for printing visible information and coded data tags on asurface substantially simultaneously, each coded data tag encoding aregion identifier for uniquely identifying a description of a region onthe surface, and a position in the region, the description associatingat least one instruction with one or more positions in the region;

a sensor for sensing at least one coded data tag and generating a signalindicative of the sensed coded data tag; and

a computer system for receiving the signal, for determining the regionidentifier and the position, for relating the position to the at leastone instruction using the description, and for executing the at leastone instruction.

Other aspects are also disclosed.

BRIEF DESCRIPTION OF DRAWINGS

Preferred and other embodiments of the invention will now be described,by way of non-limiting example only, with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic of a the relationship between a sample printednetpage and its online page description;

FIG. 2 is a schematic view of a interaction between a netpage pen, anetpage printer, a netpage page server, and a netpage applicationserver;

FIG. 3 illustrates a collection of netpage servers and printersinterconnected via a network;

FIG. 4 is a schematic view of a high-level structure of a printednetpage and its online page description;

FIG. 5 is a plan view showing a structure of a netpage tag;

FIG. 6 is a plan view showing a relationship between a set of the tagsshown in FIG. 5 and a field of view of a netpage sensing device in theform of a netpage pen;

FIG. 7 is a flowchart of a tag image processing and decoding algorithm;

FIG. 8 is a perspective view of a netpage pen and its associatedtag-sensing field-of-view cone;

FIG. 9 is a perspective exploded view of the netpage pen shown in FIG.8;

FIG. 10 is a schematic block diagram of a pen controller for the netpagepen shown in FIGS. 8 and 9;

FIG. 11 is a perspective view of a wall-mounted netpage printer;

FIG. 12 is a section through the length of the netpage printer of FIG.11;

FIG. 12 a is an enlarged portion of FIG. 12 showing a section of theduplexed print engines and glue wheel assembly;

FIG. 13 is a detailed view of the ink cartridge, ink, air and gluepaths, and print engines of the netpage printer of FIGS. 11 and 12;

FIG. 14 is a schematic block diagram of a printer controller for thenetpage printer shown in FIGS. 11 and 12;

FIG. 15 is a schematic block diagram of duplexed print enginecontrollers and Memjet™ printheads associated with the printercontroller shown in FIG. 14;

FIG. 16 is a schematic block diagram of the print engine controllershown in FIGS. 14 and 15;

FIG. 17 is a perspective view of a single Memjet™ printing element, asused in, for example, the netpage printer of FIGS. 10 to 12;

FIG. 18 is a perspective view of a small part of an array of Memjet™printing elements;

FIG. 19 is a series of perspective views illustrating the operatingcycle of the Memjet™ printing element shown in FIG. 13;

FIG. 20 is a perspective view of a short segment of a pagewidth Memjet™printhead;

FIG. 21 is a schematic view of a user class diagram;

FIG. 22 is a schematic view of a printer class diagram;

FIG. 23 is a schematic view of a pen class diagram;

FIG. 24 is a schematic view of an application class diagram;

FIG. 25 is a schematic view of a document and page description classdiagram;

FIG. 26 is a schematic view of a document and page ownership classdiagram;

FIG. 27 is a schematic view of a terminal element specialization classdiagram;

FIG. 28 is a schematic view of a static element specialization classdiagram;

FIG. 29 is a schematic view of a hyperlink element class diagram;

FIG. 30 is a schematic view of a hyperlink element specialization classdiagram;

FIG. 31 is a schematic view of a hyperlinked group class diagram;

FIG. 32 is a schematic view of a form class diagram;

FIG. 33 is a schematic view of a digital ink class diagram;

FIG. 34 is a schematic view of a field element specialization classdiagram;

FIG. 35 is a schematic view of a checkbox field class diagram;

FIG. 36 is a schematic view of a text field class diagram;

FIG. 37 is a schematic view of a signature field class diagram;

FIG. 38 is a flowchart of an input processing algorithm;

FIG. 38 a is a detailed flowchart of one step of the flowchart of FIG.38;

FIG. 39 is a schematic view of a page server command element classdiagram;

FIG. 40 is a schematic view of a resource description class diagram;

FIG. 41 is a schematic view of a favorites list class diagram;

FIG. 42 is a schematic view of a history list class diagram;

FIG. 43 is a schematic view of a subscription delivery protocol;

FIG. 44 is a schematic view of a hyperlink request class diagram;

FIG. 45 is a schematic view of a hyperlink activation protocol;

FIG. 46 is a schematic view of a form submission protocol;

FIG. 47 is a schematic view of a commission payment protocol;

FIG. 48 is a schematic view of a set of radial wedges making up asymbol;

FIG. 49 is a schematic view of a ring A and B symbol allocation scheme;

FIG. 50 is a schematic view of a first ring C and D symbol allocationscheme;

FIG. 51 is a schematic view of a second ring C and D symbol allocationscheme;

FIG. 52 is a schematic view of a triangular tag packing;

FIG. 53 is a perspective view of an icosahedron;

FIG. 54 is a perspective view of an icosahedral geodesic with frequency3;

FIG. 55 is a schematic view of a minimum tag spacing;

FIG. 56 is a schematic view of a minimum tag spacing which avoidsoverlap;

FIG. 57 is a schematic view of a first tag insertion case;

FIG. 58 is a schematic view of a second tag insertion case;

FIG. 59 is a schematic view of a third tag insertion case;

FIG. 60 is a schematic view of a fourth tag insertion case;

FIG. 61 is a schematic view of a pen orientation relative to a surface;

FIG. 62 is a schematic view of a pen pitch geometry;

FIG. 63 is a schematic view of a pen roll geometry;

FIG. 64 is a schematic view of a pen coordinate space showing physicaland optical axes of a pen;

FIG. 65 is a schematic view of a curved nib geometry;

FIG. 66 is a schematic view of an interaction between sampling frequencyand tag frequency;

FIG. 67 is a table containing equations numbered 1 to 10;

FIG. 68 is a table containing equations numbered 11 to 20;

FIG. 69 is a table containing equations numbered 21 to 26;

FIG. 70 is a table containing equations numbered 27 to 34;

FIG. 71 is a table containing equations numbered 35 to 41;

FIG. 72 is a table containing equations numbered 42 to 44;

FIG. 73 is a table containing equations numbered 45 to 47;

FIG. 74 is a table containing equations numbered 48 to 51;

FIG. 75 is a table containing equations numbered 52 to 54;

FIG. 76 is a table containing equations numbered 55 to 57;

FIG. 77 is a table containing equations numbered 58 to 59;

FIG. 78 is a table containing equations numbered 60 to 63;

FIG. 79 is a table containing equations numbered 64 to 74;

FIG. 80 is a table containing equations numbered 75 to 86;

FIG. 81 is a table containing equations numbered 87 to 99;

FIG. 82 is a table containing equations numbered 100 to 111;

FIG. 83 is a table containing equations numbered 112 to 120;

FIG. 84 is a table containing equations numbered 121 to 129;

FIG. 85 is a table containing a set of degenerate forms of equations 64to 71;

FIG. 86 is a first part of a table containing conditions and specialhandling for zero pitch and zero roll; and

FIG. 87 is a second part of the table of FIG. 86.

DETAILED DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS

Note: Memjet™ is a trade mark of Silverbrook Research Pty Ltd,Australia.

In the preferred embodiment, the invention is configured to work withthe netpage networked computer system, a detailed overview of whichfollows. It will be appreciated that not every implementation willnecessarily embody all or even most of the specific details andextensions discussed below in relation to the basic system. However, thesystem is described in its most complete form to reduce the need forexternal reference when attempting to understand the context in whichthe preferred embodiments and aspects of the present invention operate.

In brief summary, the preferred form of the netpage system employs acomputer interface in the form of a mapped surface, that is, a physicalsurface which contains references to a map of the surface maintained ina computer system. The map references can be queried by an appropriatesensing device. Depending upon the specific implementation, the mapreferences may be encoded visibly or invisibly, and defined in such away that a local query on the mapped surface yields an unambiguous mapreference both within the map and among different maps. The computersystem can contain information about features on the mapped surface, andsuch information can be retrieved based on map references supplied by asensing device used with the mapped surface. The information thusretrieved can take the form of actions which are initiated by thecomputer system on behalf of the operator in response to the operator'sinteraction with the surface features.

In its preferred form, the netpage system relies on the production of,and human interaction with, netpages. These are pages of text, graphicsand images printed on ordinary paper, but which work like interactiveweb pages. Information is encoded on each page using ink which issubstantially invisible to the unaided human eye. The ink, however, andthereby the coded data, can be sensed by an optically imaging pen andtransmitted to the netpage system.

In the preferred form, active buttons and hyperlinks on each page can beclicked with the pen to request information from the network or tosignal preferences to a network server. In one embodiment, text writtenby hand on a netpage is automatically recognized and converted tocomputer text in the netpage system, allowing forms to be filled in. Inother embodiments, signatures recorded on a netpage are automaticallyverified, allowing e-commerce transactions to be securely authorized.

As illustrated in FIG. 1, a printed netpage 1 can represent ainteractive form which can be filled in by the user both physically, onthe printed page, and “electronically”, via communication between thepen and the netpage system. The example shows a “Request” formcontaining name and address fields and a submit button. The netpageconsists of graphic data 2 printed using visible ink, and coded data 3printed as a collection of tags 4 using invisible ink. The correspondingpage description 5, stored on the netpage network, describes theindividual elements of the netpage. In particular it describes the typeand spatial extent (zone) of each interactive element (i.e. text fieldor button in the example), to allow the netpage system to correctlyinterpret input via the netpage. The submit button 6, for example, has azone 7 which corresponds to the spatial extent of the correspondinggraphic 8.

As illustrated in FIG. 2, the netpage pen 101, a preferred form of whichis shown in FIGS. 8 and 9 and described in more detail below, works inconjunction with a netpage printer 601, an Internet-connected printingappliance for home, office or mobile use. The pen is wireless andcommunicates securely with the netpage printer via a short-range radiolink 9.

The netpage printer 601, a preferred form of which is shown in FIGS. 11to 13 and described in more detail below, is able to deliver,periodically or on demand, personalized newspapers, magazines, catalogs,brochures and other publications, all printed at high quality asinteractive netpages. Unlike a personal computer, the netpage printer isan appliance which can be, for example, wall-mounted adjacent to an areawhere the morning news is first consumed, such as in a user's kitchen,near a breakfast table, or near the household's point of departure forthe day. It also comes in tabletop, desktop, portable and miniatureversions.

Netpages printed at their point of consumption combine the ease-of-useof paper with the timeliness and interactivity of an interactive medium.

As shown in FIG. 2, the netpage pen 101 interacts with the coded data ona printed netpage 1 and communicates, via a short-range radio link 9,the interaction to a netpage printer. The printer 601 sends theinteraction to the relevant netpage page server 10 for interpretation.In appropriate circumstances, the page server sends a correspondingmessage to application computer software running on a netpageapplication server 13. The application server may in turn send aresponse which is printed on the originating printer.

The netpage system is made considerably more convenient in the preferredembodiment by being used in conjunction with high-speedmicroelectromechanical system (MEMS) based inkjet (Memjet™) printers. Inthe preferred form of this technology, relatively high-speed andhigh-quality printing is made more affordable to consumers. In itspreferred form, a netpage publication has the physical characteristicsof a traditional newsmagazine, such as a set of letter-size glossy pagesprinted in full color on both sides, bound together for easy navigationand comfortable handling.

The netpage printer exploits the growing availability of broadbandInternet access. Cable service is available to 95% of households in theUnited States, and cable modem service offering broadband Internetaccess is already available to 20% of these. The netpage printer canalso operate with slower connections, but with longer delivery times andlower image quality. Indeed, the netpage system can be enabled usingexisting consumer inkjet and laser printers, although the system willoperate more slowly and will therefore be less acceptable from aconsumer's point of view. In other embodiments, the netpage system ishosted on a private intranet. In still other embodiments, the netpagesystem is hosted on a single computer or computer-enabled device, suchas a printer.

Netpage publication servers 14 on the netpage network are configured todeliver print-quality publications to netpage printers. Periodicalpublications are delivered automatically to subscribing netpage printersvia pointcasting and multicasting Internet protocols. Personalizedpublications are filtered and formatted according to individual userprofiles.

A netpage printer can be configured to support any number of pens, and apen can work with any number of netpage printers. In the preferredimplementation, each netpage pen has a unique identifier. A householdmay have a collection of colored netpage pens, one assigned to eachmember of the family. This allows each user to maintain a distinctprofile with respect to a netpage publication server or applicationserver.

A netpage pen can also be registered with a netpage registration server11 and linked to one or more payment card accounts. This allowse-commerce payments to be securely authorized using the netpage pen. Thenetpage registration server compares the signature captured by thenetpage pen with a previously registered signature, allowing it toauthenticate the user's identity to an e-commerce server. Otherbiometrics can also be used to verify identity. A version of the netpagepen includes fingerprint scanning, verified in a similar way by thenetpage registration server.

Although a netpage printer may deliver periodicals such as the morningnewspaper without user intervention, it can be configured never todeliver unsolicited junk mail. In its preferred form, it only deliversperiodicals from subscribed or otherwise authorized sources. In thisrespect, the netpage printer is unlike a fax machine or e-mail accountwhich is visible to any junk mailer who knows the telephone number oremail address.

1 Netpage System Architecture

Each object model in the system is described using a Unified ModelingLanguage (UML) class diagram. A class diagram consists of a set ofobject classes connected by relationships, and two kinds ofrelationships are of interest here: associations and generalizations. Anassociation represents some kind of relationship between objects, i.e.between instances of classes. A generalization relates actual classes,and can be understood in the following way: if a class is thought of asthe set of all objects of that class, and class A is a generalization ofclass B, then B is simply a subset of A. The UML does not directlysupport second-order modelling—i.e. classes of classes.

Each class is drawn as a rectangle labelled with the name of the class.It contains a list of the attributes of the class, separated from thename by a horizontal line, and a list of the operations of the class,separated from the attribute list by a horizontal line. In the classdiagrams which follow, however, operations are never modelled.

An association is drawn as a line joining two classes, optionallylabelled at either end with the multiplicity of the association. Thedefault multiplicity is one. An asterisk (*) indicates a multiplicity of“many”, i.e. zero or more. Each association is optionally labelled withits name, and is also optionally labelled at either end with the role ofthe corresponding class. An open diamond indicates an aggregationassociation (“is-part-of”), and is drawn at the aggregator end of theassociation line.

A generalization relationship (“is-a”) is drawn as a solid line joiningtwo classes, with an arrow (in the form of an open triangle) at thegeneralization end.

When a class diagram is broken up into multiple diagrams, any classwhich is duplicated is shown with a dashed outline in all but the maindiagram which defines it. It is shown with attributes only where it isdefined.

1.1 Netpages

Netpages are the foundation on which a netpage network is built. Theyprovide a paper-based user interface to published information andinteractive services.

A netpage consists of a printed page (or other surface region) invisiblytagged with references to an online description of the page. The onlinepage description is maintained persistently by a netpage page server.The page description describes the visible layout and content of thepage, including text, graphics and images. It also describes the inputelements on the page, including buttons, hyperlinks, and input fields. Anetpage allows markings made with a netpage pen on its surface to besimultaneously captured and processed by the netpage system.

Multiple netpages can share the same page description. However, to allowinput through otherwise identical pages to be distinguished, eachnetpage is assigned a unique page identifier. This page ID hassufficient precision to distinguish between a very large number ofnetpages.

Each reference to the page description is encoded in a printed tag. Thetag identifies the unique page on which it appears, and therebyindirectly identifies the page description. The tag also identifies itsown position on the page. Characteristics of the tags are described inmore detail below.

Tags are printed in infrared-absorptive ink on any substrate which isinfrared-reflective, such as ordinary paper. Near-infrared wavelengthsare invisible to the human eye but are easily sensed by a solid-stateimage sensor with an appropriate filter.

A tag is sensed by an area image sensor in the netpage pen, and the tagdata is transmitted to the netpage system via the nearest netpageprinter. The pen is wireless and communicates with the netpage printervia a short-range radio link. Tags are sufficiently small and denselyarranged that the pen can reliably image at least one tag even on asingle click on the page. It is important that the pen recognize thepage ID and position on every interaction with the page, since theinteraction is stateless. Tags are error-correctably encoded to makethem partially tolerant to surface damage.

The netpage page server maintains a unique page instance for eachprinted netpage, allowing it to maintain a distinct set of user-suppliedvalues for input fields in the page description for each printednetpage.

The relationship between the page description, the page instance, andthe printed netpage is shown in FIG. 4. The page instance is associatedwith both the netpage printer which printed it and, if known, thenetpage user who requested it.

1.2 Netpage Tags

1.2.1 Tag Data Content

In a preferred form, each tag identifies the region in which it appears,and the location of that tag within the region. A tag may also containflags which relate to the region as a whole or to the tag. One or moreflag bits may, for example, signal a tag sensing device to providefeedback indicative of a function associated with the immediate area ofthe tag, without the sensing device having to refer to a description ofthe region. A netpage pen may, for example, illuminate an “active area”LED when in the zone of a hyperlink.

As will be more clearly explained below, in a preferred embodiment, eachtag contains an easily recognized invariant structure which aids initialdetection, and which assists in minimizing the effect of any warpinduced by the surface or by the sensing process. The tags preferablytile the entire page, and are sufficiently small and densely arrangedthat the pen can reliably image at least one tag even on a single clickon the page. It is important that the pen recognize the page ID andposition on every interaction with the page, since the interaction isstateless.

In a preferred embodiment, the region to which a tag refers coincideswith an entire page, and the region ID encoded in the tag is thereforesynonymous with the page ID of the page on which the tag appears. Inother embodiments, the region to which a tag refers can be an arbitrarysubregion of a page or other surface. For example, it can coincide withthe zone of an interactive element, in which case the region ID candirectly identify the interactive element.

TABLE 1 Tag data Field Precision (bits) Region ID 100 Tag ID 16 Flags 4Total 120

Each tag contains 120 bits of information, typically allocated as shownin Table 1. Assuming a maximum tag density of 64 per square inch, a16-bit tag ID supports a region size of up to 1024 square inches. Largerregions can be mapped continuously without increasing the tag IDprecision simply by using abutting regions and maps. The 100-bit regionID allows 2¹⁰⁰ (˜10³⁰ or a million trillion trillion) different regionsto be uniquely identified.

1.2.2 Tag Data Encoding

The 120 bits of tag data are redundantly encoded using a (15, 5)Reed-Solomon code. This yields 360 encoded bits consisting of 6codewords of 15 4-bit symbols each. The (15, 5) code allows up to 5symbol errors to be corrected per codeword, i.e. it is tolerant of asymbol error rate of up to 33% per codeword.

Each 4-bit symbol is represented in a spatially coherent way in the tag,and the symbols of the six codewords are interleaved spatially withinthe tag. This ensures that a burst error (an error affecting multiplespatially adjacent bits) damages a minimum number of symbols overall anda minimum number of symbols in any one codeword, thus maximising thelikelihood that the burst error can be fully corrected.

1.2.3 Physical Tag Structure

The physical representation of the tag, shown in FIG. 5, includes fixedtarget structures 15, 16, 17 and variable data areas 18. The fixedtarget structures allow a sensing device such as the netpage pen todetect the tag and infer its three-dimensional orientation relative tothe sensor. The data areas contain representations of the individualbits of the encoded tag data.

To achieve proper tag reproduction, the tag is rendered at a resolutionof 256×256 dots. When printed at 1600 dots per inch this yields a tagwith a diameter of about 4 mm. At this resolution the tag is designed tobe surrounded by a “quiet area” of radius 16 dots. Since the quiet areais also contributed by adjacent tags, it only adds 16 dots to theeffective diameter of the tag.

The tag includes six target structures: a detection ring 15; anorientation axis target 16; and four perspective targets 17.

The detection ring 15 allows the sensing device to initially detect thetag 4. The ring is easy to detect because it is rotationally invariantand because a simple correction of its aspect ratio removes most of theeffects of perspective distortion. The orientation axis 16 allows thesensing device to determine the approximate planar orientation of thetag due to the yaw of the sensor. The orientation axis is skewed toyield a unique orientation. The four perspective targets 17 allow thesensing device to infer an accurate two-dimensional perspectivetransform of the tag and hence an accurate three-dimensional positionand orientation of the tag relative to the sensor.

All target structures are redundantly large to improve their immunity tonoise.

The overall tag shape is circular. This supports, amongst other things,optimal tag packing on an irregular triangular grid. In combination withthe circular detection ring 15, this makes a circular arrangement ofdata bits within the tag optimal. To maximise its size, each data bit isrepresented by a radial wedge 510 in the form of an area bounded by tworadial lines 512, a radially inner arc 514 and a radially outer arc 516.Each wedge 510 has a minimum dimension of 8 dots at 1600 dpi and isdesigned so that its base (i.e. its inner arc 514), is at least equal tothis minimum dimension. The radial height of the wedge 510 is alwaysequal to the minimum dimension. Each 4-bit data symbol is represented byan array 518 of 2×2 wedges 510, as best shown in FIG. 48.

The 15 4-bit data symbols of each of the six codewords are allocated tothe four concentric symbol rings 18 a to 18 d, shown in FIG. 5, ininterleaved fashion as shown in FIGS. 49 to 51. Symbols of first tosixth codewords 520-525 are allocated alternately in circularprogression around the tag.

The interleaving is designed to maximise the average spatial distancebetween any two symbols of the same codeword.

In order to support “single-click” interaction with a tagged region viaa sensing device, the sensing device must be able to see at least oneentire tag in its field of view no matter where in the region or at whatorientation it is positioned. The required diameter of the field of viewof the sensing device is therefore a function of the size and spacing ofthe tags.

Assuming a circular tag shape, the minimum diameter of the sensor fieldof view is obtained when the tags are tiled on a equilateral triangulargrid, as shown in FIG. 6.

1.2.4 Tag Image Processing and Decoding

The tag image processing and decoding performed by a sensing device suchas the netpage pen is shown in FIG. 7. While a captured image is beingacquired from the image sensor, the dynamic range of the image isdetermined (at 20). The center of the range is then chosen as the binarythreshold for the image 21. The image is then thresholded and segmentedinto connected pixel regions (i.e. shapes 23) (at 22). Shapes which aretoo small to represent tag target structures are discarded. The size andcentroid of each shape is also computed.

Binary shape moments 25 are then computed (at 24) for each shape, andthese provide the basis for subsequently locating target structures.Central shape moments are by their nature invariant of position, and canbe easily made invariant of scale, aspect ratio and rotation.

The ring target structure 15 is the first to be located (at 26). A ringhas the advantage of being very well behaved when perspective-distorted.Matching proceeds by aspect-normalizing and rotation-normalizing eachshape's moments. Once its second-order moments are normalized the ringis easy to recognize even if the perspective distortion was significant.The ring's original aspect and rotation 27 together provide a usefulapproximation of the perspective transform.

The axis target structure 16 is the next to be located (at 28). Matchingproceeds by applying the ring's normalizations to each shape's moments,and rotation-normalizing the resulting moments. Once its second-ordermoments are normalized the axis target is easily recognized. Note thatone third order moment is required to disambiguate the two possibleorientations of the axis. The shape is deliberately skewed to one sideto make this possible. Note also that it is only possible torotation-normalize the axis target after it has had the ring'snormalizations applied, since the perspective distortion can hide theaxis target's axis. The axis target's original rotation provides auseful approximation of the tag's rotation due to pen yaw 29.

The four perspective target structures 17 are the last to be located (at30). Good estimates of their positions are computed based on their knownspatial relationships to the ring and axis targets, the aspect androtation of the ring, and the rotation of the axis. Matching proceeds byapplying the ring's normalizations to each shape's moments. Once theirsecond-order moments are normalized the circular perspective targets areeasy to recognize, and the target closest to each estimated position istaken as a match. The original centroids of the four perspective targetsare then taken to be the perspective-distorted corners 31 of a square ofknown size in tag space, and an eight-degree-of-freedom perspectivetransform 33 is inferred (at 32) based on solving the well-understoodequations relating the four tag-space and image-space point pairs.

The inferred tag-space to image-space perspective transform is used toproject (at 36) each known data bit position in tag space into imagespace where the real-valued position is used to bilinearly interpolate(at 36) the four relevant adjacent pixels in the input image. Thepreviously computed image threshold 21 is used to threshold the resultto produce the final bit value 37.

Once all 360 data bits 37 have been obtained in this way, each of thesix 60-bit Reed-Solomon codewords is decoded (at 38) to yield 20 decodedbits 39, or 120 decoded bits in total. Note that the codeword symbolsare sampled in codeword order, so that codewords are implicitlyde-interleaved during the sampling process.

The ring target 15 is only sought in a subarea of the image whoserelationship to the image guarantees that the ring, if found, is part ofa complete tag. If a complete tag is not found and successfully decoded,then no pen position is recorded for the current frame. Given adequateprocessing power and ideally a non-minimal field of view 193, analternative strategy involves seeking another tag in the current image.

The obtained tag data indicates the identity of the region containingthe tag and the position of the tag within the region. An accurateposition 35 of the pen nib in the region, as well as the overallorientation 35 of the pen, is then inferred (at 34) from the perspectivetransform 33 observed on the tag and the known spatial relationshipbetween the pen's physical axis and the pen's optical axis.

1.2.5 Tag Map

Decoding a tag results in a region ID, a tag ID, and a tag-relative pentransform. Before the tag ID and the tag-relative pen location can betranslated into an absolute location within the tagged region, thelocation of the tag within the region must be known. This is given by atag map, a function which maps each tag ID in a tagged region to acorresponding location. The tag map class diagram is shown in FIG. 22,as part of the netpage printer class diagram.

A tag map reflects the scheme used to tile the surface region with tags,and this can vary according to surface type. When multiple taggedregions share the same tiling scheme and the same tag numbering scheme,they can also share the same tag map.

The tag map for a region must be retrievable via the region ID. Thus,given a region ID, a tag ID and a pen transform, the tag map can beretrieved, the tag ID can be translated into an absolute tag locationwithin the region, and the tag-relative pen location can be added to thetag location to yield an absolute pen location within the region.

1.2.6 Tagging Schemes

Two distinct surface coding schemes are of interest, both of which usethe tag structure described earlier in this section. The preferredcoding scheme uses “location-indicating” tags as already discussed. Analternative coding scheme uses object-indicating tags.

A location-indicating tag contains a tag ID which, when translatedthrough the tag map associated with the tagged region, yields a uniquetag location within the region. The tag-relative location of the pen isadded to this tag location to yield the location of the pen within theregion. This in turn is used to determine the location of the penrelative to a user interface element in the page description associatedwith the region. Not only is the user interface element itselfidentified, but a location relative to the user interface element isidentified. Location-indicating tags therefore trivially support thecapture of an absolute pen path in the zone of a particular userinterface element.

An object-indicating tag contains a tag ID which directly identifies auser interface element in the page description associated with theregion. All the tags in the zone of the user interface element identifythe user interface element, making them all identical and thereforeindistinguishable. Object-indicating tags do not, therefore, support thecapture of an absolute pen path. They do, however, support the captureof a relative pen path. So long as the position sampling frequencyexceeds twice the encountered tag frequency, the displacement from onesampled pen position to the next within a stroke can be unambiguouslydetermined.

Assume a sampling wavelength of λ_(S) and a tag wavelength of λ_(T),with a relationship as defined in EQ 38. For two adjacent positionsamples P_(i) and P_(i+1), one of EQ 39 and EQ 40 will hold.

Assuming both equations hold leads to the relationship defined in EQ 41.

Since EQ 41 contradicts EQ 38, the assumption that both EQ 39 and EQ 40hold must be incorrect, and the choice is therefore unambiguous, asstated.

The illustration in FIG. 60 shows four tags 500 and a one-dimensionalstroke of six sample positions 582 which satisfy EQ 38. Possible aliases584 of the sample positions are also shown. From inspection, if thedistance from one sample position to the next is λ_(S), then thedistance from a sample position to the alias of the next sample positionexceeds λ_(S).

If the tag wavelength λ_(T) is 4.7 mm, as discussed in earlier, then thesampling wavelength λ_(S) must be less than 2.35 mm. If the temporalsampling frequency is 100 Hz as required for accurate handwritingrecognition, then the pen speed must be less than 235 mm/s to satisfy EQ38.

With either tagging scheme, the tags function in cooperation withassociated visual elements on the netpage as user interactive elementsin that a user can interact with the printed page using an appropriatesensing device in order for tag data to be read by the sensing deviceand for an appropriate response to be generated in the netpage system.

1.3 Document and Page Descriptions

A preferred embodiment of a document and page description class diagramis shown in FIGS. 25 and 26.

In the netpage system a document is described at three levels. At themost abstract level the document 836 has a hierarchical structure whoseterminal elements 839 are associated with content objects 840 such astext objects, text style objects, image objects, etc. Once the documentis printed on a printer with a particular page size and according to aparticular user's scale factor preference, the document is paginated andotherwise formatted. Formatted terminal elements 835 will in some casesbe associated with content objects which are different from thoseassociated with their corresponding terminal elements, particularlywhere the content objects are style-related. Each printed instance of adocument and page is also described separately, to allow input capturedthrough a particular page instance 830 to be recorded separately frominput captured through other instances of the same page description.

The presence of the most abstract document description on the pageserver allows a user to request a copy of a document without beingforced to accept the source document's specific format. The user may berequesting a copy through a printer with a different page size, forexample. Conversely, the presence of the formatted document descriptionon the page server allows the page server to efficiently interpret useractions on a particular printed page.

A formatted document 834 consists of a set of formatted pagedescriptions 5, each of which consists of a set of formatted terminalelements 835. Each formatted element has a spatial extent or zone 58 onthe page. This defines the active area of input elements such ashyperlinks and input fields.

A document instance 831 corresponds to a formatted document 834. Itconsists of a set of page instances 830, each of which corresponds to apage description 5 of the formatted document. Each page instance 830describes a single unique printed netpage 1, and records the page ID 50of the netpage. A page instance is not part of a document instance if itrepresents a copy of a page requested in isolation.

A page instance consists of a set of terminal element instances 832. Anelement instance only exists if it records instance-specificinformation. Thus, a hyperlink instance exists for a hyperlink elementbecause it records a transaction ID 55 which is specific to the pageinstance, and a field instance exists for a field element because itrecords input specific to the page instance. An element instance doesnot exist, however, for static elements such as textflows.

A terminal element can be a static element 843, a hyperlink element 844,a field element 845 or a page server command element 846, as shown inFIG. 27. A static element 843 can be a style element 847 with anassociated style object 854, a textflow element 848 with an associatedstyled text object 855, an image element 849 with an associated imageelement 856, a graphic element 850 with an associated graphic object857, a video clip element 851 with an associated video clip object 858,an audio clip element 852 with an associated audio clip object 859, or ascript element 853 with an associated script object 860, as shown inFIG. 28.

A page instance has a background field 833 which is used to record anydigital ink captured on the page which does not apply to a specificinput element.

In the preferred form of the invention, a tag map 811 is associated witheach page instance to allow tags on the page to be translated intolocations on the page.

1.4 The Netpage Network

In a preferred embodiment, a netpage network consists of a distributedset of netpage page servers 10, netpage registration servers 11, netpageID servers 12, netpage application servers 13, netpage publicationservers 14, and netpage printers 601 connected via a network 19 such asthe Internet, as shown in FIG. 3.

The netpage registration server 11 is a server which recordsrelationships between users, pens, printers, applications andpublications, and thereby authorizes various network activities. Itauthenticates users and acts as a signing proxy on behalf ofauthenticated users in application transactions. It also provideshandwriting recognition services. As described above, a netpage pageserver 10 maintains persistent information about page descriptions andpage instances. The netpage network includes any number of page servers,each handling a subset of page instances. Since a page server alsomaintains user input values for each page instance, clients such asnetpage printers send netpage input directly to the appropriate pageserver. The page server interprets any such input relative to thedescription of the corresponding page.

A netpage ID server 12 allocates document IDs 51 on demand, and providesload-balancing of page servers via its ID allocation scheme.

A netpage printer uses the Internet Distributed Name System (DNS), orsimilar, to resolve a netpage page ID 50 into the network address of thenetpage page server handling the corresponding page instance.

A netpage application server 13 is a server which hosts interactivenetpage applications. A netpage publication server 14 is an applicationserver which publishes netpage documents to netpage printers. They aredescribed in detail in Section 2.

Netpage servers can be hosted on a variety of network server platformsfrom manufacturers such as IBM, Hewlett-Packard, and Sun. Multiplenetpage servers can run concurrently on a single host, and a singleserver can be distributed over a number of hosts. Some or all of thefunctionality provided by netpage servers, and in particular thefunctionality provided by the ID server and the page server, can also beprovided directly in a netpage appliance such as a netpage printer, in acomputer workstation, or on a local network.

1.5 The Netpage Printer

The netpage printer 601 is an appliance which is registered with thenetpage system and prints netpage documents on demand and viasubscription. Each printer has a unique printer ID 62, and is connectedto the netpage network via a network such as the Internet, ideally via abroadband connection.

Apart from identity and security settings in non-volatile memory, thenetpage printer contains no persistent storage. As far as a user isconcerned, “the network is the computer”. Netpages functioninteractively across space and time with the help of the distributednetpage page servers 10, independently of particular netpage printers.

The netpage printer receives subscribed netpage documents from netpagepublication servers 14. Each document is distributed in two parts: thepage layouts, and the actual text and image objects which populate thepages. Because of personalization, page layouts are typically specificto a particular subscriber and so are pointcast to the subscriber'sprinter via the appropriate page server. Text and image objects, on theother hand, are typically shared with other subscribers, and so aremulticast to all subscribers' printers and the appropriate page servers.

The netpage publication server optimizes the segmentation of documentcontent into pointcasts and multicasts. After receiving the pointcast ofa document's page layouts, the printer knows which multicasts, if any,to listen to.

Once the printer has received the complete page layouts and objects thatdefine the document to be printed, it can print the document.

The printer rasterizes and prints odd and even pages simultaneously onboth sides of the sheet. It contains duplexed print engine controllers760 and print engines utilizing Memjet™ printheads 350 for this purpose.

The printing process consists of two decoupled stages: rasterization ofpage descriptions, and expansion and printing of page images. The rasterimage processor (RIP) consists of one or more standard DSPs 757 runningin parallel. The duplexed print engine controllers consist of customprocessors which expand, dither and print page images in real time,synchronized with the operation of the printheads in the print engines.

Printers not enabled for IR printing have the option to print tags usingIR-absorptive black ink, although this restricts tags to otherwise emptyareas of the page. Although such pages have more limited functionalitythan IR-printed pages, they are still classed as netpages.

A normal netpage printer prints netpages on sheets of paper. Morespecialised netpage printers may print onto more specialised surfaces,such as globes. Each printer supports at least one surface type, andsupports at least one tag tiling scheme, and hence tag map, for eachsurface type. The tag map 811 which describes the tag tiling schemeactually used to print a document becomes associated with that documentso that the document's tags can be correctly interpreted.

FIG. 2 shows the netpage printer class diagram, reflectingprinter-related information maintained by a registration server 11 onthe netpage network.

A preferred embodiment of the netpage printer is described in greaterdetail in Section 6 below, with reference to FIGS. 11 to 16.

1.5.1 Memjet™ Printheads

The netpage system can operate using printers made with a wide range ofdigital printing technologies, including thermal inkjet, piezoelectricinkjet, laser electrophotographic, and others. However, for wideconsumer acceptance, it is desirable that a netpage printer have thefollowing characteristics:

-   -   photographic quality color printing    -   high quality text printing    -   high reliability    -   low printer cost    -   low ink cost    -   low paper cost    -   simple operation    -   nearly silent printing    -   high printing speed    -   simultaneous double sided printing    -   compact form factor    -   low power consumption

No commercially available printing technology has all of thesecharacteristics.

To enable to production of printers with these characteristics, thepresent applicant has invented a new print technology, referred to asMemjet™ technology. Memjet™ is a drop-on-demand inkjet technology thatincorporates pagewidth printheads fabricated usingmicroelectromechanical systems (MEMS) technology. FIG. 17 shows a singleprinting element 300 of a Memjet™ printhead. The netpage wallprinterincorporates 168960 printing elements 300 to form a 1600 dpi pagewidthduplex printer. This printer simultaneously prints cyan, magenta,yellow, black, and infrared inks as well as paper conditioner and inkfixative.

The printing element 300 is approximately 110 microns long by 32 micronswide. Arrays of these printing elements are formed on a siliconsubstrate 301 that incorporates CMOS logic, data transfer, timing, anddrive circuits (not shown).

Major elements of the printing element 300 are the nozzle 302, thenozzle rim 303, the nozzle chamber 304, the fluidic seal 305, the inkchannel rim 306, the lever arm 307, the active actuator beam pair 308,the passive actuator beam pair 309, the active actuator anchor 310, thepassive actuator anchor 311, and the ink inlet 312.

The active actuator beam pair 308 is mechanically joined to the passiveactuator beam pair 309 at the join 319. Both beams pairs are anchored attheir respective anchor points 310 and 311. The combination of elements308, 309, 310, 311, and 319 form a cantilevered electrothermal bendactuator 320.

FIG. 18 shows a small part of an array of printing elements 300,including a cross section 315 of a printing element 300. The crosssection 315 is shown without ink, to clearly show the ink inlet 312 thatpasses through the silicon wafer 301.

FIGS. 19( a), 19(b) and 19(c) show the operating cycle of a Memjet™printing element 300.

FIG. 19( a) shows the quiescent position of the ink meniscus 316 priorto printing an ink droplet. Ink is retained in the nozzle chamber bysurface tension at the ink meniscus 316 and at the fluidic seal 305formed between the nozzle chamber 304 and the ink channel rim 306.

While printing, the printhead CMOS circuitry distributes data from theprint engine controller to the correct printing element, latches thedata, and buffers the data to drive the electrodes 318 of the activeactuator beam pair 308. This causes an electrical current to passthrough the beam pair 308 for about one microsecond, resulting in Jouleheating. The temperature increase resulting from Joule heating causesthe beam pair 308 to expand. As the passive actuator beam pair 309 isnot heated, it does not expand, resulting in a stress difference betweenthe two beam pairs. This stress difference is partially resolved by thecantilevered end of the electrothermal bend actuator 320 bending towardsthe substrate 301. The lever arm 307 transmits this movement to thenozzle chamber 304. The nozzle chamber 304 moves about two microns tothe position shown in FIG. 19( b). This increases the ink pressure,forcing ink 321 out of the nozzle 302, and causing the ink meniscus 316to bulge. The nozzle rim 303 prevents the ink meniscus 316 fromspreading across the surface of the nozzle chamber 304.

As the temperature of the beam pairs 308 and 309 equalizes, the actuator320 returns to its original position. This aids in the break-off of theink droplet 317 from the ink 321 in the nozzle chamber, as shown in FIG.19( c). The nozzle chamber is refilled by the action of the surfacetension at the meniscus 316.

FIG. 20 shows a segment of a printhead 350. In a netpage printer, thelength of the printhead is the full width of the paper (typically 210mm) in the direction 351. The segment shown is 0.4 mm long (about 0.2%of a complete printhead). When printing, the paper is moved past thefixed printhead in the direction 352. The printhead has 6 rows ofinterdigitated printing elements 300, printing the six colors or typesof ink supplied by the ink inlets 312.

To protect the fragile surface of the printhead during operation, anozzle guard wafer 330 is attached to the printhead substrate 301. Foreach nozzle 302 there is a corresponding nozzle guard hole 331 throughwhich the ink droplets are fired. To prevent the nozzle guard holes 331from becoming blocked by paper fibers or other debris, filtered air ispumped through the air inlets 332 and out of the nozzle guard holesduring printing. To prevent ink 321 from drying, the nozzle guard issealed while the printer is idle.

1.6 The Netpage Pen

The active sensing device of the netpage system is typically a pen 101,which, using its embedded controller 134, is able to capture and decodeIR position tags from a page via an image sensor. The image sensor is asolid-state device provided with an appropriate filter to permit sensingat only near-infrared wavelengths. As described in more detail below,the system is able to sense when the nib is in contact with the surface,and the pen is able to sense tags at a sufficient rate to capture humanhandwriting (i.e. at 200 dpi or greater and 100 Hz or faster).Information captured by the pen is encrypted and wirelessly transmittedto the printer (or base station), the printer or base stationinterpreting the data with respect to the (known) page structure.

The preferred embodiment of the netpage pen operates both as a normalmarking ink pen and as a non-marking stylus. The marking aspect,however, is not necessary for using the netpage system as a browsingsystem, such as when it is used as an Internet interface. Each netpagepen is registered with the netpage system and has a unique pen ID 61.FIG. 23 shows the netpage pen class diagram, reflecting pen-relatedinformation maintained by a registration server 11 on the netpagenetwork.

When either nib is in contact with a netpage, the pen determines itsposition and orientation relative to the page. The nib is attached to aforce sensor, and the force on the nib is interpreted relative to athreshold to indicate whether the pen is “up” or “down”. This allows ainteractive element on the page to be ‘clicked’ by pressing with the pennib, in order to request, say, information from a network. Furthermore,the force is captured as a continuous value to allow, say, the fulldynamics of a signature to be verified.

The pen determines the position and orientation of its nib on thenetpage by imaging, in the infrared spectrum, an area 193 of the page inthe vicinity of the nib. It decodes the nearest tag and computes theposition of the nib relative to the tag from the observed perspectivedistortion on the imaged tag and the known geometry of the pen optics.Although the position resolution of the tag may be low, because the tagdensity on the page is inversely proportional to the tag size, theadjusted position resolution is quite high, exceeding the minimumresolution required for accurate handwriting recognition.

Pen actions relative to a netpage are captured as a series of strokes. Astroke consists of a sequence of time-stamped pen positions on the page,initiated by a pen-down event and completed by the subsequent pen-upevent. A stroke is also tagged with the page ID 50 of the netpagewhenever the page ID changes, which, under normal circumstances, is atthe commencement of the stroke.

Each netpage pen has a current selection 826 associated with it,allowing the user to perform copy and paste operations etc. Theselection is timestamped to allow the system to discard it after adefined time period. The current selection describes a region of a pageinstance. It consists of the most recent digital ink stroke capturedthrough the pen relative to the background area of the page. It isinterpreted in an application-specific manner once it is submitted to anapplication via a selection hyperlink activation.

Each pen has a current nib 824. This is the nib last notified by the pento the system. In the case of the default netpage pen described above,either the marking black ink nib or the non-marking stylus nib iscurrent. Each pen also has a current nib style 825. This is the nibstyle last associated with the pen by an application, e.g. in responseto the user selecting a color from a palette. The default nib style isthe nib style associated with the current nib. Strokes captured througha pen are tagged with the current nib style. When the strokes aresubsequently reproduced, they are reproduced in the nib style with whichthey are tagged.

Whenever the pen is within range of a printer with which it cancommunicate, the pen slowly flashes its “online” LED. When the pen failsto decode a stroke relative to the page, it momentarily activates its“error” LED. When the pen succeeds in decoding a stroke relative to thepage, it momentarily activates its “ok” LED.

A sequence of captured strokes is referred to as digital ink. Digitalink forms the basis for the digital exchange of drawings andhandwriting, for online recognition of handwriting, and for onlineverification of signatures.

The pen is wireless and transmits digital ink to the netpage printer viaa short-range radio link. The transmitted digital ink is encrypted forprivacy and security and packetized for efficient transmission, but isalways flushed on a pen-up event to ensure timely handling in theprinter.

When the pen is out-of-range of a printer it buffers digital ink ininternal memory, which has a capacity of over ten minutes of continuoushandwriting. When the pen is once again within range of a printer, ittransfers any buffered digital ink.

A pen can be registered with any number of printers, but because allstate data resides in netpages both on paper and on the network, it islargely immaterial which printer a pen is communicating with at anyparticular time.

A preferred embodiment of the pen is described in greater detail inSection 6 below, with reference to FIGS. 8 to 10.

1.7 Netpage Interaction

The netpage printer 601 receives data relating to a stroke from the pen101 when the pen is used to interact with a netpage 1. The coded data 3of the tags 4 is read by the pen when it is used to execute a movement,such as a stroke. The data allows the identity of the particular pageand associated interactive element to be determined and an indication ofthe relative positioning of the pen relative to the page to be obtained.The indicating data is transmitted to the printer, where it resolves,via the DNS, the page ID 50 of the stroke into the network address ofthe netpage page server 10 which maintains the corresponding pageinstance 830. It then transmits the stroke to the page server. If thepage was recently identified in an earlier stroke, then the printer mayalready have the address of the relevant page server in its cache. Eachnetpage consists of a compact page layout maintained persistently by anetpage page server (see below). The page layout refers to objects suchas images, fonts and pieces of text, typically stored elsewhere on thenetpage network.

When the page server receives the stroke from the pen, it retrieves thepage description to which the stroke applies, and determines whichelement of the page description the stroke intersects. It is then ableto interpret the stroke in the context of the type of the relevantelement.

A “click” is a stroke where the distance and time between the pen downposition and the subsequent pen up position are both less than somesmall maximum. An object which is activated by a click typicallyrequires a click to be activated, and accordingly, a longer stroke isignored. The failure of a pen action, such as a “sloppy” click, toregister is indicated by the lack of response from the pen's “ok” LED.

There are two kinds of input elements in a netpage page description:hyperlinks and form fields. Input through a form field can also triggerthe activation of an associated hyperlink.

1.7.1 Hyperlinks

A hyperlink is a means of sending a message to a remote application, andtypically elicits a printed response in the netpage system.

A hyperlink element 844 identifies the application 71 which handlesactivation of the hyperlink, a link ID 54 which identifies the hyperlinkto the application, an “alias required” flag which asks the system toinclude the user's application alias ID 65 in the hyperlink activation,and a description which is used when the hyperlink is recorded as afavorite or appears in the user's history. The hyperlink element classdiagram is shown in FIG. 29.

When a hyperlink is activated, the page server sends a request to anapplication somewhere on the network. The application is identified byan application ID 64, and the application ID is resolved in the normalway via the DNS. There are three types of hyperlinks: general hyperlinks863, form hyperlinks 865, and selection hyperlinks 864, as shown in FIG.30. A general hyperlink can implement a request for a linked document,or may simply signal a preference to a server. A form hyperlink submitsthe corresponding form to the application. A selection hyperlink submitsthe current selection to the application. If the current selectioncontains a single-word piece of text, for example, the application mayreturn a single-page document giving the word's meaning within thecontext in which it appears, or a translation into a different language.Each hyperlink type is characterized by what information is submitted tothe application.

The corresponding hyperlink instance 862 records a transaction ID 55which can be specific to the page instance on which the hyperlinkinstance appears. The transaction ID can identify user-specific data tothe application, for example a “shopping cart” of pending purchasesmaintained by a purchasing application on behalf of the user.

The system includes the pen's current selection 826 in a selectionhyperlink activation. The system includes the content of the associatedform instance 868 in a form hyperlink activation, although if thehyperlink has its “submit delta” attribute set, only input since thelast form submission is included. The system includes an effectivereturn path in all hyperlink activations.

A hyperlinked group 866 is a group element 838 which has an associatedhyperlink, as shown in FIG. 31. When input occurs through any fieldelement in the group, the hyperlink 844 associated with the group isactivated. A hyperlinked group can be used to associate hyperlinkbehavior with a field such as a checkbox. It can also be used, inconjunction with the “submit delta” attribute of a form hyperlink, toprovide continuous input to an application. It can therefore be used tosupport a “blackboard” interaction model, i.e. where input is capturedand therefore shared as soon as it occurs.

1.7.2 Forms

A form defines a collection of related input fields used to capture arelated set of inputs through a printed netpage. A form allows a user tosubmit one or more parameters to an application software program runningon a server.

A form 867 is a group element 838 in the document hierarchy. Itultimately contains a set of terminal field elements 839. A forminstance 868 represents a printed instance of a form. It consists of aset of field instances 870 which correspond to the field elements 845 ofthe form.

Each field instance has an associated value 871, whose type depends onthe type of the corresponding field element. Each field value recordsinput through a particular printed form instance, i.e. through one ormore printed netpages. The form class diagram is shown in FIG. 32.

Each form instance has a status 872 which indicates whether the form isactive, frozen, submitted, void or expired. A form is active when firstprinted. A form becomes frozen once it is signed. A form becomessubmitted once one of its submission hyperlinks has been activated,unless the hyperlink has its “submit delta” attribute set. A formbecomes void when the user invokes a void form, reset form or duplicateform page command. A form expires when the time the form has been activeexceeds the form's specified lifetime. While the form is active, forminput is allowed. Input through a form which is not active is insteadcaptured in the background field 833 of the relevant page instance. Whenthe form is active or frozen, form submission is allowed. Any attempt tosubmit a form when the form is not active or frozen is rejected, andinstead elicits an form status report.

Each form instance is associated (at 59) with any form instances derivedfrom it, thus providing a version history. This allows all but thelatest version of a form in a particular time period to be excluded froma search.

All input is captured as digital ink. Digital ink 873 consists of a setof timestamped stroke groups 874, each of which consists of a set ofstyled strokes 875. Each stroke consists of a set of timestamped penpositions 876, each of which also includes pen orientation and nibforce. The digital ink class diagram is shown in FIG. 33.

A field element 845 can be a checkbox field 877, a text field 878, adrawing field 879, or a signature field 880. The field element classdiagram is shown in FIG. 34. Any digital ink captured in a field's zone58 is assigned to the field.

A checkbox field has an associated boolean value 881, as shown in FIG.35. Any mark (a tick, a cross, a stroke, a fill zigzag, etc.) capturedin a checkbox field's zone causes a true value to be assigned to thefield's value.

A text field has an associated text value 882, as shown in FIG. 36. Anydigital ink captured in a text field's zone is automatically convertedto text via online handwriting recognition, and the text is assigned tothe field's value. Online handwriting recognition is well-understood(see for example Tappert, C., C. Y. Suen and T. Wakahara, “The State ofthe Art in On-Line Handwriting Recognition”, IEEE Transactions onPattern Analysis and Machine Intelligence, Vol. 12, No. 8, August 1990).

A signature field has an associated digital signature value 883, asshown in FIG. 37. Any digital ink captured in a signature field's zoneis automatically verified with respect to the identity of the owner ofthe pen, and a digital signature of the content of the form of which thefield is part is generated and assigned to the field's value. Thedigital signature is generated using the pen user's private signaturekey specific to the application which owns the form. Online signatureverification is well-understood (see for example Plamondon, R. and G.Lorette, “Automatic Signature Verification and Writer Identification—TheState of the Art”, Pattern Recognition, Vol. 22, No. 2, 1989).

A field element is hidden if its “hidden” attribute is set. A hiddenfield element does not have an input zone on a page and does not acceptinput. It can have an associated field value which is included in theform data when the form containing the field is submitted.

“Editing” commands, such as strike-throughs indicating deletion, canalso be recognized in form fields.

Because the handwriting recognition algorithm works “online” (i.e. withaccess to the dynamics of the pen movement), rather than “offline” (i.e.with access only to a bitmap of pen markings), it can recognize run-ondiscretely-written characters with relatively high accuracy, without awriter-dependent training phase. A writer-dependent model of handwritingis automatically generated over time, however, and can be generatedup-front if necessary,

Digital ink, as already stated, consists of a sequence of strokes. Anystroke which starts in a particular element's zone is appended to thatelement's digital ink stream, ready for interpretation. Any stroke notappended to an object's digital ink stream is appended to the backgroundfield's digital ink stream.

Digital ink captured in the background field is interpreted as aselection gesture. Circumscription of one or more objects is generallyinterpreted as a selection of the circumscribed objects, although theactual interpretation is application-specific.

Table 2 summarises these various pen interactions with a netpage.

TABLE 2 Summary of pen interactions with a netpage Object Type Pen inputAction Hyperlink General Click Submit action to application Form ClickSubmit form to application Selection Click Submit selection toapplication Form field Checkbox Any mark Assign true to field TextHandwriting Convert digital ink to text; assign text to field DrawingDigital ink Assign digital ink to field Signature Signature Verifydigital ink signature; generate digital signature of form; assigndigital signature to field None — Circumscription Assign digital ink tocurrent selection

The system maintains a current selection for each pen. The selectionconsists simply of the most recent stroke captured in the backgroundfield. The selection is cleared after an inactivity timeout to ensurepredictable behavior.

The raw digital ink captured in every field is retained on the netpagepage server and is optionally transmitted with the form data when theform is submitted to the application. This allows the application tointerrogate the raw digital ink should it suspect the originalconversion, such as the conversion of handwritten text. This can, forexample, involve human intervention at the application level for formswhich fail certain application-specific consistency checks. As anextension to this, the entire background area of a form can bedesignated as a drawing field. The application can then decide, on thebasis of the presence of digital ink outside the explicit fields of theform, to route the form to a human operator, on the assumption that theuser may have indicated amendments to the filled-in fields outside ofthose fields.

FIG. 38 shows a flowchart of the process of handling pen input relativeto a netpage. The process consists of receiving (at 884) a stroke fromthe pen; identifying (at 885) the page instance 830 to which the page ID50 in the stroke refers; retrieving (at 886) the page description 5;identifying (at 887) a formatted element 839 whose zone 58 the strokeintersects; determining (at 888) whether the formatted elementcorresponds to a field element, and if so appending (at 892) thereceived stroke to the digital ink of the field value 871, interpreting(at 893) the accumulated digital ink of the field, and determining (at894) whether the field is part of a hyperlinked group 866 and if soactivating (at 895) the associated hyperlink; alternatively determining(at 889) whether the formatted element corresponds to a hyperlinkelement and if so activating (at 895) the corresponding hyperlink;alternatively, in the absence of an input field or hyperlink, appending(at 890) the received stroke to the digital ink of the background field833; and copying (at 891) the received stroke to the current selection826 of the current pen, as maintained by the registration server.

FIG. 38 a shows a detailed flowchart of step 893 in the process shown inFIG. 38, where the accumulated digital ink of a field is interpretedaccording to the type of the field. The process consists of determining(at 896) whether the field is a checkbox and (at 897) whether thedigital ink represents a checkmark, and if so assigning (at 898) a truevalue to the field value; alternatively determining (at 899) whether thefield is a text field and if so converting (at 900) the digital ink tocomputer text, with the help of the appropriate registration server, andassigning (at 901) the converted computer text to the field value;alternatively determining (at 902) whether the field is a signaturefield and if so verifying (at 903) the digital ink as the signature ofthe pen's owner, with the help of the appropriate registration server,creating (at 904) a digital signature of the contents of thecorresponding form, also with the help of the registration server andusing the pen owner's private signature key relating to thecorresponding application, and assigning (at 905) the digital signatureto the field value.

1.7.3 Page Server Commands

A page server command is a command which is handled locally by the pageserver. It operates directly on form, page and document instances.

A page server command 907 can be a void form command 908, a duplicateform command 909, a reset form command 910, a get form status command911, a duplicate page command 912, a reset page command 913, a get pagestatus command 914, a duplicate document command 915, a reset documentcommand 916, or a get document status command 917, as shown in FIG. 39.

A void form command voids the corresponding form instance. A duplicateform command voids the corresponding form instance and then produces anactive printed copy of the current form instance with field valuespreserved. The copy contains the same hyperlink transaction IDs as theoriginal, and so is indistinguishable from the original to anapplication. A reset form command voids the corresponding form instanceand then produces an active printed copy of the form instance with fieldvalues discarded. A get form status command produces a printed report onthe status of the corresponding form instance, including who publishedit, when it was printed, for whom it was printed, and the form status ofthe form instance.

Since a form hyperlink instance contains a transaction ID, theapplication has to be involved in producing a new form instance. Abutton requesting a new form instance is therefore typically implementedas a hyperlink.

A duplicate page command produces a printed copy of the correspondingpage instance with the background field value preserved. If the pagecontains a form or is part of a form, then the duplicate page command isinterpreted as a duplicate form command. A reset page command produces aprinted copy of the corresponding page instance with the backgroundfield value discarded. If the page contains a form or is part of a form,then the reset page command is interpreted as a reset form command. Aget page status command produces a printed report on the status of thecorresponding page instance, including who published it, when it wasprinted, for whom it was printed, and the status of any forms itcontains or is part of.

The netpage logo which appears on every netpage is usually associatedwith a duplicate page element.

When a page instance is duplicated with field values preserved, fieldvalues are printed in their native form, i.e. a checkmark appears as astandard checkmark graphic, and text appears as typeset text. Onlydrawings and signatures appear in their original form, with a signatureaccompanied by a standard graphic indicating successful signatureverification.

A duplicate document command produces a printed copy of thecorresponding document instance with background field values preserved.If the document contains any forms, then the duplicate document commandduplicates the forms in the same way a duplicate form command does. Areset document command produces a printed copy of the correspondingdocument instance with background field values discarded. If thedocument contains any forms, then the reset document command resets theforms in the same way a reset form command does. A get document statuscommand produces a printed report on the status of the correspondingdocument instance, including who published it, when it was printed, forwhom it was printed, and the status of any forms it contains.

If the page server command's “on selected” attribute is set, then thecommand operates on the page identified by the pen's current selectionrather than on the page containing the command. This allows a menu ofpage server commands to be printed. If the target page doesn't contain apage server command element for the designated page server command, thenthe command is ignored.

An application can provide application-specific handling by embeddingthe relevant page server command element in a hyperlinked group. Thepage server activates the hyperlink associated with the hyperlinkedgroup rather than executing the page server command.

A page server command element is hidden if its “hidden” attribute isset. A hidden command element does not have an input zone on a page andso cannot be activated directly by a user. It can, however, be activatedvia a page server command embedded in a different page, if that pageserver command has its “on selected” attribute set.

1.8 Standard Features of Netpages

In the preferred form, each netpage is printed with the netpage logo atthe bottom to indicate that it is a netpage and therefore hasinteractive properties. The logo also acts as a copy button. In mostcases pressing the logo produces a copy of the page. In the case of aform, the button produces a copy of the entire form. And in the case ofa secure document, such as a ticket or coupon, the button elicits anexplanatory note or advertising page.

The default single-page copy function is handled directly by therelevant netpage page server. Special copy functions are handled bylinking the logo button to an application.

1.9 User Help System

In a preferred embodiment, the netpage printer has a single buttonlabelled “Help”. When pressed it elicits a single page of information,including:

-   -   status of printer connection    -   status of printer consumables    -   top-level help menu    -   document function menu    -   top-level netpage network directory

The help menu provides a hierarchical manual on how to use the netpagesystem.

The document function menu includes the following functions:

-   -   print a copy of a document    -   print a clean copy of a form    -   print the status of a document

A document function is initiated by simply pressing the button and thentouching any page of the document. The status of a document indicateswho published it and when, to whom it was delivered, and to whom andwhen it was subsequently submitted as a form.

The netpage network directory allows the user to navigate the hierarchyof publications and services on the network. As an alternative, the usercan call the netpage network “900” number “yellow pages” and speak to ahuman operator. The operator can locate the desired document and routeit to the user's printer. Depending on the document type, the publisheror the user pays the small “yellow pages” service fee.

The help page is obviously unavailable if the printer is unable toprint. In this case the “error” light is lit and the user can requestremote diagnosis over the network.

2 Personalized Publication Model

In the following description, news is used as a canonical publicationexample to illustrate personalization mechanisms in the netpage system.Although news is often used in the limited sense of newspaper andnewsmagazine news, the intended scope in the present context is wider.

In the netpage system, the editorial content and the advertising contentof a news publication are personalized using different mechanisms. Theeditorial content is personalized according to the reader's explicitlystated and implicitly captured interest profile. The advertising contentis personalized according to the reader's locality and demographic.

2.1 Editorial Personalization

A subscriber can draw on two kinds of news sources: those that delivernews publications, and those that deliver news streams. While newspublications are aggregated and edited by the publisher, news streamsare aggregated either by a news publisher or by a specialized newsaggregator. News publications typically correspond to traditionalnewspapers and newsmagazines, while news streams can be many and varied:a “raw” news feed from a news service, a cartoon strip, a freelancewriter's column, a friend's bulletin board, or the reader's own e-mail.

The netpage publication server supports the publication of edited newspublications as well as the aggregation of multiple news streams. Byhandling the aggregation and hence the formatting of news streamsselected directly by the reader, the server is able to place advertisingon pages over which it otherwise has no editorial control.

The subscriber builds a daily newspaper by selecting one or morecontributing news publications, and creating a personalized version ofeach. The resulting daily editions are printed and bound together into asingle newspaper. The various members of a household typically expresstheir different interests and tastes by selecting different dailypublications and then customizing them.

For each publication, the reader optionally selects specific sections.Some sections appear daily, while others appear weekly. The dailysections available from The New York Times online, for example, include“Page One Plus”, “National”, “International”, “Opinion”, “Business”,“Arts/Living”, “Technology”, and “Sports”. The set of available sectionsis specific to a publication, as is the default subset.

The reader can extend the daily newspaper by creating custom sections,each one drawing on any number of news streams. Custom sections might becreated for e-mail and friends' announcements (“Personal”), or formonitoring news feeds for specific topics (“Alerts” or “Clippings”).

For each section, the reader optionally specifies its size, eitherqualitatively (e.g. short, medium, or long), or numerically (i.e. as alimit on its number of pages), and the desired proportion ofadvertising, either qualitatively (e.g. high, normal, low, none), ornumerically (i.e. as a percentage).

The reader also optionally expresses a preference for a large number ofshorter articles or a small number of longer articles. Each article isideally written (or edited) in both short and long forms to support thispreference.

An article may also be written (or edited) in different versions tomatch the expected sophistication of the reader, for example to providechildren's and adults' versions. The appropriate version is selectedaccording to the reader's age. The reader can specify a “reading age”which takes precedence over their biological age.

The articles which make up each section are selected and prioritized bythe editors, and each is assigned a useful lifetime. By default they aredelivered to all relevant subscribers, in priority order, subject tospace constraints in the subscribers' editions.

In sections where it is appropriate, the reader may optionally enablecollaborative filtering. This is then applied to articles which have asufficiently long lifetime. Each article which qualifies forcollaborative filtering is printed with rating buttons at the end of thearticle. The buttons can provide an easy choice (e.g. “liked” and“disliked”), making it more likely that readers will bother to rate thearticle.

Articles with high priorities and short lifetimes are thereforeeffectively considered essential reading by the editors and aredelivered to most relevant subscribers.

The reader optionally specifies a serendipity factor, eitherqualitatively (e.g. do or don't surprise me), or numerically. A highserendipity factor lowers the threshold used for matching duringcollaborative filtering. A high factor makes it more likely that thecorresponding section will be filled to the reader's specified capacity.A different serendipity factor can be specified for different days ofthe week.

The reader also optionally specifies topics of particular interestwithin a section, and this modifies the priorities assigned by theeditors.

The speed of the reader's Internet connection affects the quality atwhich images can be delivered. The reader optionally specifies apreference for fewer images or smaller images or both. If the number orsize of images is not reduced, then images may be delivered at lowerquality (i.e. at lower resolution or with greater compression).

At a global level, the reader specifies how quantities, dates, times andmonetary values are localized. This involves specifying whether unitsare imperial or metric, a local timezone and time format, and a localcurrency, and whether the localization consist of in situ translation orannotation. These preferences are derived from the reader's locality bydefault.

To reduce reading difficulties caused by poor eyesight, the readeroptionally specifies a global preference for a larger presentation. Bothtext and images are scaled accordingly, and less information isaccommodated on each page.

The language in which a news publication is published, and itscorresponding text encoding, is a property of the publication and not apreference expressed by the user. However, the netpage system can beconfigured to provide automatic translation services in various guises.

2.2 Advertising Localization and Targeting

The personalization of the editorial content directly affects theadvertising content, because advertising is typically placed to exploitthe editorial context. Travel ads, for example, are more likely toappear in a travel section than elsewhere. The value of the editorialcontent to an advertiser (and therefore to the publisher) lies in itsability to attract large numbers of readers with the right demographics.

Effective advertising is placed on the basis of locality anddemographics. Locality determines proximity to particular services,retailers etc., and particular interests and concerns associated withthe local community and environment. Demographics determine generalinterests and preoccupations as well as likely spending patterns.

A news publisher's most profitable product is advertising “space”, amulti-dimensional entity determined by the publication's geographiccoverage, the size of its readership, its readership demographics, andthe page area available for advertising.

In the netpage system, the netpage publication server computes theapproximate multi-dimensional size of a publication's saleableadvertising space on a per-section basis, taking into account thepublication's geographic coverage, the section's readership, the size ofeach reader's section edition, each reader's advertising proportion, andeach reader's demographic.

In comparison with other media, the netpage system allows theadvertising space to be defined in greater detail, and allows smallerpieces of it to be sold separately. It therefore allows it to be sold atcloser to its true value.

For example, the same advertising “slot” can be sold in varyingproportions to several advertisers, with individual readers' pagesrandomly receiving the advertisement of one advertiser or another,overall preserving the proportion of space sold to each advertiser.

The netpage system allows advertising to be linked directly to detailedproduct information and online purchasing. It therefore raises theintrinsic value of the advertising space.

Because personalization and localization are handled automatically bynetpage publication servers, an advertising aggregator can providearbitrarily broad coverage of both geography and demographics. Thesubsequent disaggregation is efficient because it is automatic. Thismakes it more cost-effective for publishers to deal with advertisingaggregators than to directly capture advertising. Even though theadvertising aggregator is taking a proportion of advertising revenue,publishers may find the change profit-neutral because of the greaterefficiency of aggregation. The advertising aggregator acts as anintermediary between advertisers and publishers, and may place the sameadvertisement in multiple publications.

It is worth noting that ad placement in a netpage publication can bemore complex than ad placement in the publication's traditionalcounterpart, because the publication's advertising space is morecomplex. While ignoring the full complexities of negotiations betweenadvertisers, advertising aggregators and publishers, the preferred formof the netpage system provides some automated support for thesenegotiations, including support for automated auctions of advertisingspace. Automation is particularly desirable for the placement ofadvertisements which generate small amounts of income, such as small orhighly localized advertisements.

Once placement has been negotiated, the aggregator captures and editsthe advertisement and records it on a netpage ad server.Correspondingly, the publisher records the ad placement on the relevantnetpage publication server. When the netpage publication server lays outeach user's personalized publication, it picks the relevantadvertisements from the netpage ad server.

2.3 User Profiles

2.3.1 Information Filtering

The personalization of news and other publications relies on anassortment of user-specific profile information, including:

-   -   publication customizations    -   collaborative filtering vectors    -   contact details    -   presentation preferences

The customization of a publication is typically publication-specific,and so the customization information is maintained by the relevantnetpage publication server.

A collaborative filtering vector consists of the user's ratings of anumber of news items. It is used to correlate different users' interestsfor the purposes of making recommendations. Although there are benefitsto maintaining a single collaborative filtering vector independently ofany particular publication, there are two reasons why it is morepractical to maintain a separate vector for each publication: there islikely to be more overlap between the vectors of subscribers to the samepublication than between those of subscribers to different publications;and a publication is likely to want to present its users' collaborativefiltering vectors as part of the value of its brand, not to be foundelsewhere. Collaborative filtering vectors are therefore also maintainedby the relevant netpage publication server.

Contact details, including name, street address, ZIP Code, state,country, telephone numbers, are global by nature, and are maintained bya netpage registration server.

Presentation preferences, including those for quantities, dates andtimes, are likewise global and maintained in the same way.

The localization of advertising relies on the locality indicated in theuser's contact details, while the targeting of advertising relies onpersonal information such as date of birth, gender, marital status,income, profession, education, or qualitative derivatives such as agerange and income range.

For those users who choose to reveal personal information foradvertising purposes, the information is maintained by the relevantnetpage registration server. In the absence of such information,advertising can be targeted on the basis of the demographic associatedwith the user's ZIP or ZIP+4 Code.

Each user, pen, printer, application provider and application isassigned its own unique identifier, and the netpage registration servermaintains the relationships between them, as shown in FIGS. 21, 22, 23and 24. For registration purposes, a publisher is a special kind ofapplication provider, and a publication is a special kind ofapplication.

Each user 800 may be authorized to use any number of printers 802, andeach printer may allow any number of users to use it. Each user has asingle default printer (at 66), to which periodical publications aredelivered by default, whilst pages printed on demand are delivered tothe printer through which the user is interacting. The server keepstrack of which publishers a user has authorized to print to the user'sdefault printer. A publisher does not record the ID of any particularprinter, but instead resolves the ID when it is required.

When a user subscribes 808 to a publication 807, the publisher 806 (i.e.application provider 803) is authorized to print to a specified printeror the user's default printer. This authorization can be revoked at anytime by the user. Each user may have several pens 801, but a pen isspecific to a single user. If a user is authorized to use a particularprinter, then that printer recognizes any of the user's pens.

The pen ID is used to locate the corresponding user profile maintainedby a particular netpage registration server, via the DNS in the usualway.

A Web terminal 809 can be authorized to print on a particular netpageprinter, allowing Web pages and netpage documents encountered during Webbrowsing to be conveniently printed on the nearest netpage printer.

The netpage system can collect, on behalf of a printer provider, feesand commissions on income earned through publications printed on theprovider's printers. Such income can include advertising fees,click-through fees, e-commerce commissions, and transaction fees. If theprinter is owned by the user, then the user is the printer provider.

Each user also has a netpage account 820 which is used to accumulatemicro-debits and credits (such as those described in the precedingparagraph); contact details 815, including name, address and telephonenumbers; global preferences 816, including privacy, delivery andlocalization settings; any number of biometric records 817, containingthe user's encoded signature 818, fingerprint 819 etc; a handwritingmodel 819 automatically maintained by the system; and SET payment cardaccounts 821 with which e-commerce payments can be made.

2.3.2 Favorites List

A netpage user can maintain a list 922 of “favorites”—links to usefuldocuments etc. on the netpage network. The list is maintained by thesystem on the user's behalf. It is organized as a hierarchy of folders924, a preferred embodiment of which is shown in the class diagram inFIG. 41.

2.3.3 History List

The system maintains a history list 929 on each user's behalf,containing links to documents etc. accessed by the user through thenetpage system. It is organized as a date-ordered list, a preferredembodiment of which is shown in the class diagram in FIG. 42.

2.4 Intelligent Page Layout

The netpage publication server automatically lays out the pages of eachuser's personalized publication on a section-by-section basis. Sincemost advertisements are in the form of pre-formatted rectangles, theyare placed on the page before the editorial content.

The advertising ratio for a section can be achieved with wildly varyingadvertising ratios on individual pages within the section, and the adlayout algorithm exploits this. The algorithm is configured to attemptto co-locate closely tied editorial and advertising content, such asplacing ads for roofing material specifically within the publicationbecause of a special feature on do-it-yourself roofing repairs.

The editorial content selected for the user, including text andassociated images and graphics, is then laid out according to variousaesthetic rules.

The entire process, including the selection of ads and the selection ofeditorial content, must be iterated once the layout has converged, toattempt to more closely achieve the user's stated section sizepreference. The section size preference can, however, be matched onaverage over time, allowing significant day-to-day variations.

2.5 Document Format

Once the document is laid out, it is encoded for efficient distributionand persistent storage on the netpage network.

The primary efficiency mechanism is the separation of informationspecific to a single user's edition and information shared betweenmultiple users' editions. The specific information consists of the pagelayout. The shared information consists of the objects to which the pagelayout refers, including images, graphics, and pieces of text.

A text object contains fully-formatted text represented in theExtensible Markup Language (XML) using the Extensible StylesheetLanguage (XSL). XSL provides precise control over text formattingindependently of the region into which the text is being set, which inthis case is being provided by the layout. The text object containsembedded language codes to enable automatic translation, and embeddedhyphenation hints to aid with paragraph formatting.

An image object encodes an image in the JPEG 2000 wavelet-basedcompressed image format. A graphic object encodes a 2D graphic inScalable Vector Graphics (SVG) format.

The layout itself consists of a series of placed image and graphicobjects, linked textflow objects through which text objects flow,hyperlinks and input fields as described above, and watermark regions.These layout objects are summarized in Table 3. The layout uses acompact format suitable for efficient distribution and storage.

TABLE 3 Netpage layout objects Layout object Attribute Format of linkedobject Image Position — Image object ID JPEG 2000 Graphic Position —Graphic object ID SVG Textflow Textflow ID — Zone — Optional text objectID XML/XSL Hyperlink Type — Zone — Application ID, etc. — Field Type —Meaning — Zone — Watermark Zone —2.6 Document Distribution

As described above, for purposes of efficient distribution andpersistent storage on the netpage network, a user-specific page layoutis separated from the shared objects to which it refers.

When a subscribed publication is ready to be distributed, the netpagepublication server allocates, with the help of the netpage ID server 12,a unique ID for each page, page instance, document, and documentinstance.

The server computes a set of optimized subsets of the shared content andcreates a multicast channel for each subset, and then tags eachuser-specific layout with the names of the multicast channels which willcarry the shared content used by that layout. The server then pointcastseach user's layouts to that user's printer via the appropriate pageserver, and when the pointcasting is complete, multicasts the sharedcontent on the specified channels. After receiving its pointcast, eachpage server and printer subscribes to the multicast channels specifiedin the page layouts. During the multicasts, each page server and printerextracts from the multicast streams those objects referred to by itspage layouts. The page servers persistently archive the received pagelayouts and shared content.

Once a printer has received all the objects to which its page layoutsrefer, the printer re-creates the fully-populated layout and thenrasterizes and prints it.

Under normal circumstances, the printer prints pages faster than theycan be delivered. Assuming a quarter of each page is covered withimages, the average page has a size of less than 400 KB. The printer cantherefore hold in excess of 100 such pages in its internal 64 MB memory,allowing for temporary buffers etc. The printer prints at a rate of onepage per second. This is equivalent to 400 KB or about 3 Mbit of pagedata per second, which is similar to the highest expected rate of pagedata delivery over a broadband network.

Even under abnormal circumstances, such as when the printer runs out ofpaper, it is likely that the user will be able to replenish the papersupply before the printer's 100-page internal storage capacity isexhausted.

However, if the printer's internal memory does fill up, then the printerwill be unable to make use of a multicast when it first occurs. Thenetpage publication server therefore allows printers to submit requestsfor re-multicasts. When a critical number of requests is received or atimeout occurs, the server re-multicasts the corresponding sharedobjects.

Once a document is printed, a printer can produce an exact duplicate atany time by retrieving its page layouts and contents from the relevantpage server.

2.7 On-Demand Documents

When a netpage document is requested on demand, it can be personalizedand delivered in much the same way as a periodical. However, since thereis no shared content, delivery is made directly to the requestingprinter without the use of multicast.

When a non-netpage document is requested on demand, it is notpersonalized, and it is delivered via a designated netpage formattingserver which reformats it as a netpage document. A netpage formattingserver is a special instance of a netpage publication server. Thenetpage formatting server has knowledge of various Internet documentformats, including Adobe's Portable Document Format (PDF), and HypertextMarkup Language (HTML). In the case of HTML, it can make use of thehigher resolution of the printed page to present Web pages in amulti-column format, with a table of contents. It can automaticallyinclude all Web pages directly linked to the requested page. The usercan tune this behavior via a preference.

The netpage formatting server makes standard netpage behavior, includinginteractivity and persistence, available on any Internet document, nomatter what its origin and format. It hides knowledge of differentdocument formats from both the netpage printer and the netpage pageserver, and hides knowledge of the netpage system from Web servers.

3 Security

3.1 Cryptography

Cryptography is used to protect sensitive information, both in storageand in transit, and to authenticate parties to a transaction. There aretwo classes of cryptography in widespread use: secret-key cryptographyand public-key cryptography. The netpage network uses both classes ofcryptography.

Secret-key cryptography, also referred to as symmetric cryptography,uses the same key to encrypt and decrypt a message. Two parties wishingto exchange messages must first arrange to securely exchange the secretkey.

Public-key cryptography, also referred to as asymmetric cryptography,uses two encryption keys. The two keys are mathematically related insuch a way that any message encrypted using one key can only bedecrypted using the other key. One of these keys is then published,while the other is kept private. The public key is used to encrypt anymessage intended for the holder of the private key. Once encrypted usingthe public key, a message can only be decrypted using the private key.Thus two parties can securely exchange messages without first having toexchange a secret key. To ensure that the private key is secure, it isnormal for the holder of the private key to generate the key pair.

Public-key cryptography can be used to create a digital signature. Theholder of the private key can create a known hash of a message and thenencrypt the hash using the private key. Anyone can then verify that theencrypted hash constitutes the “signature” of the holder of the privatekey with respect to that particular message by decrypting the encryptedhash using the public key and verifying the hash against the message. Ifthe signature is appended to the message, then the recipient of themessage can verify both that the message is genuine and that it has notbeen altered in transit.

To make public-key cryptography work, there has to be a way todistribute public keys which prevents impersonation. This is normallydone using certificates and certificate authorities. A certificateauthority is a trusted third party which authenticates the connectionbetween a public key and someone's identity. The certificate authorityverifies the person's identity by examining identity documents, and thencreates and signs a digital certificate containing the person's identitydetails and public key. Anyone who trusts the certificate authority canuse the public key in the certificate with a high degree of certaintythat it is genuine. They just have to verify that the certificate hasindeed been signed by the certificate authority, whose public key iswell-known.

In most transaction environments, public-key cryptography is only usedto create digital signatures and to securely exchange secret sessionkeys. Secret-key cryptography is used for all other purposes.

In the following discussion, when reference is made to the securetransmission of information between a netpage printer and a server, whatactually happens is that the printer obtains the server's certificate,authenticates it with reference to the certificate authority, uses thepublic key-exchange key in the certificate to exchange a secret sessionkey with the server, and then uses the secret session key to encrypt themessage data. A session key, by definition, can have an arbitrarilyshort lifetime.

3.2 Netpage Printer Security

Each netpage printer is assigned a pair of unique identifiers at time ofmanufacture which are stored in read-only memory in the printer and inthe netpage registration server database. The first ID 62 is public anduniquely identifies the printer on the netpage network. The second ID issecret and is used when the printer is first registered on the network.

When the printer connects to the netpage network for the first timeafter installation, it creates a signature public/private key pair. Ittransmits the secret ID and the public key securely to the netpageregistration server. The server compares the secret ID against theprinter's secret ID recorded in its database, and accepts theregistration if the IDs match. It then creates and signs a certificatecontaining the printer's public ID and public signature key, and storesthe certificate in the registration database.

The netpage registration server acts as a certificate authority fornetpage printers, since it has access to secret information allowing itto verify printer identity.

When a user subscribes to a publication, a record is created in thenetpage registration server database authorizing the publisher to printthe publication to the user's default printer or a specified printer.Every document sent to a printer via a page server is addressed to aparticular user and is signed by the publisher using the publisher'sprivate signature key. The page server verifies, via the registrationdatabase, that the publisher is authorized to deliver the publication tothe specified user. The page server verifies the signature using thepublisher's public key, obtained from the publisher's certificate storedin the registration database.

The netpage registration server accepts requests to add printingauthorizations to the database, so long as those requests are initiatedvia a pen registered to the printer.

3.3 Netpage Pen Security

Each netpage pen is assigned a unique identifier at time of manufacturewhich is stored in read-only memory in the pen and in the netpageregistration server database. The pen ID 61 uniquely identifies the penon the netpage network.

A netpage pen can “know” a number of netpage printers, and a printer can“know” a number of pens. A pen communicates with a printer via a radiofrequency signal whenever it is within range of the printer. Once a penand printer are registered, they regularly exchange session keys.Whenever the pen transmits digital ink to the printer, the digital inkis always encrypted using the appropriate session key. Digital ink isnever transmitted in the clear.

A pen stores a session key for every printer it knows, indexed byprinter ID, and a printer stores a session key for every pen it knows,indexed by pen ID. Both have a large but finite storage capacity forsession keys, and will forget a session key on a least-recently-usedbasis if necessary.

When a pen comes within range of a printer, the pen and printer discoverwhether they know each other. If they don't know each other, then theprinter determines whether it is supposed to know the pen. This mightbe, for example, because the pen belongs to a user who is registered touse the printer. If the printer is meant to know the pen but doesn't,then it initiates the automatic pen registration procedure. If theprinter isn't meant to know the pen, then it agrees with the pen toignore it until the pen is placed in a charging cup, at which time itinitiates the registration procedure.

In addition to its public ID, the pen contains a secret key-exchangekey. The key-exchange key is also recorded in the netpage registrationserver database at time of manufacture. During registration, the pentransmits its pen ID to the printer, and the printer transmits the penID to the netpage registration server. The server generates a sessionkey for the printer and pen to use, and securely transmits the sessionkey to the printer. It also transmits a copy of the session keyencrypted with the pen's key-exchange key. The printer stores thesession key internally, indexed by the pen ID, and transmits theencrypted session key to the pen. The pen stores the session keyinternally, indexed by the printer ID.

Although a fake pen can impersonate a pen in the pen registrationprotocol, only a real pen can decrypt the session key transmitted by theprinter.

When a previously unregistered pen is first registered, it is of limiteduse until it is linked to a user. A registered but “un-owned” pen isonly allowed to be used to request and fill in netpage user and penregistration forms, to register a new user to which the new pen isautomatically linked, or to add a new pen to an existing user.

The pen uses secret-key rather than public-key encryption because ofhardware performance constraints in the pen.

3.4 Secure Documents

The netpage system supports the delivery of secure documents such astickets and coupons. The netpage printer includes a facility to printwatermarks, but will only do so on request from publishers who aresuitably authorized. The publisher indicates its authority to printwatermarks in its certificate, which the printer is able toauthenticate.

The “watermark” printing process uses an alternative dither matrix inspecified “watermark” regions of the page. Back-to-back pages containmirror-image watermark regions which coincide when printed. The dithermatrices used in odd and even pages' watermark regions are designed toproduce an interference effect when the regions are viewed together,achieved by looking through the printed sheet.

The effect is similar to a watermark in that it is not visible whenlooking at only one side of the page, and is lost when the page iscopied by normal means.

Pages of secure documents cannot be copied using the built-in netpagecopy mechanism described in Section 1.9 above. This extends to copyingnetpages on netpage-aware photocopiers.

Secure documents are typically generated as part of e-commercetransactions. They can therefore include the user's photograph which wascaptured when the user registered biometric information with the netpageregistration server, as described in Section 2.

When presented with a secure netpage document, the recipient can verifyits authenticity by requesting its status in the usual way. The uniqueID of a secure document is only valid for the lifetime of the document,and secure document IDs are allocated non-contiguously to prevent theirprediction by opportunistic forgers. A secure document verification pencan be developed with built-in feedback on verification failure, tosupport easy point-of-presentation document verification.

Clearly neither the watermark nor the user's photograph are secure in acryptographic sense. They simply provide a significant obstacle tocasual forgery. Online document verification, particularly using averification pen, provides an added level of security where it isneeded, but is still not entirely immune to forgeries.

3.5 Non-Repudiation

In the netpage system, forms submitted by users are delivered reliablyto forms handlers and are persistently archived on netpage page servers.It is therefore impossible for recipients to repudiate delivery.

E-commerce payments made through the system, as described in Section 4,are also impossible for the payee to repudiate.

4 Electronic Commerce Model

4.1 Secure Electronic Transaction (SET)

The netpage system uses the Secure Electronic Transaction (SET) systemas one of its payment systems. SET, having been developed by MasterCardand Visa, is organized around payment cards, and this is reflected inthe terminology. However, much of the system is independent of the typeof accounts being used.

In SET, cardholders and merchants register with a certificate authorityand are issued with certificates containing their public signature keys.The certificate authority verifies a cardholder's registration detailswith the card issuer as appropriate, and verifies a merchant'sregistration details with the acquirer as appropriate. Cardholders andmerchants store their respective private signature keys securely ontheir computers. During the payment process, these certificates are usedto mutually authenticate a merchant and cardholder, and to authenticatethem both to the payment gateway.

SET has not yet been adopted widely, partly because cardholdermaintenance of keys and certificates is considered burdensome. Interimsolutions which maintain cardholder keys and certificates on a serverand give the cardholder access via a password have met with somesuccess.

4.2 Set Payments

In the netpage system the netpage registration server acts as a proxyfor the netpage user (i.e. the cardholder) in SET payment transactions.

The netpage system uses biometrics to authenticate the user andauthorize SET payments. Because the system is pen-based, the biometricused is the user's on-line signature, consisting of time-varying penposition and pressure. A fingerprint biometric can also be used bydesigning a fingerprint sensor into the pen, although at a higher cost.The type of biometric used only affects the capture of the biometric,not the authorization aspects of the system.

The first step to being able to make SET payments is to register theuser's biometric with the netpage registration server. This is done in acontrolled environment, for example a bank, where the biometric can becaptured at the same time as the user's identity is verified. Thebiometric is captured and stored in the registration database, linked tothe user's record. The user's photograph is also optionally captured andlinked to the record. The SET cardholder registration process iscompleted, and the resulting private signature key and certificate arestored in the database. The user's payment card information is alsostored, giving the netpage registration server enough information to actas the user's proxy in any SET payment transaction.

When the user eventually supplies the biometric to complete a payment,for example by signing a netpage order form, the printer securelytransmits the order information, the pen ID and the biometric data tothe netpage registration server. The server verifies the biometric withrespect to the user identified by the pen ID, and from then on acts asthe user's proxy in completing the SET payment transaction.

4.3 Micro-Payments

The netpage system includes a mechanism for micro-payments, to allow theuser to be conveniently charged for printing low-cost documents ondemand and for copying copyright documents, and possibly also to allowthe user to be reimbursed for expenses incurred in printing advertisingmaterial. The latter depends on the level of subsidy already provided tothe user.

When the user registers for e-commerce, a network account is establishedwhich aggregates micro-payments. The user receives a statement on aregular basis, and can settle any outstanding debit balance using thestandard payment mechanism.

The network account can be extended to aggregate subscription fees forperiodicals, which would also otherwise be presented to the user in theform of individual statements.

4.4 Transactions

When a user requests a netpage in a particular application context, theapplication is able to embed a user-specific transaction ID 55 in thepage. Subsequent input through the page is tagged with the transactionID, and the application is thereby able to establish an appropriatecontext for the user's input.

When input occurs through a page which is not user-specific, however,the application must use the user's unique identity to establish acontext. A typical example involves adding items from a pre-printedcatalog page to the user's virtual “shopping cart”. To protect theuser's privacy, however, the unique user ID 60 known to the netpagesystem is not divulged to applications. This is to prevent differentapplication providers from easily correlating independently accumulatedbehavioral data.

The netpage registration server instead maintains an anonymousrelationship between a user and an application via a unique alias ID 65,as shown in FIG. 24. Whenever the user activates a hyperlink tagged withthe “registered” attribute, the netpage page server asks the netpageregistration server to translate the associated application ID 64,together with the pen ID 61, into an alias ID 65. The alias ID is thensubmitted to the hyperlink's application.

The application maintains state information indexed by alias ID, and isable to retrieve user-specific state information without knowledge ofthe global identity of the user.

The system also maintains an independent certificate and privatesignature key for each of a user's applications, to allow it to signapplication transactions on behalf of the user using onlyapplication-specific information.

To assist the system in routing product bar code (UPC) “hyperlink”activations, the system records a favorite application on behalf of theuser for any number of product types.

Each application is associated with an application provider, and thesystem maintains an account on behalf of each application provider, toallow it to credit and debit the provider for click-through fees etc.

An application provider can be a publisher of periodical subscribedcontent. The system records the user's willingness to receive thesubscribed publication, as well as the expected frequency ofpublication.

4.5 Resource Descriptions and Copyright

A preferred embodiment of a resource description class diagram is shownin FIG. 40.

Each document and content object may be described by one or moreresource descriptions 842. Resource descriptions use the Dublin Coremetadata element set, which is designed to facilitate discovery ofelectronic resources. Dublin Core metadata conforms to the World WideWeb Consortium (W3C) Resource Description Framework (RDF).

A resource description may identify rights holders 920. The netpagesystem automatically transfers copyright fees from users to rightsholders when users print copyright content.

5 Communications Protocols

A communications protocol defines an ordered exchange of messagesbetween entities. In the netpage system, entities such as pens, printersand servers utilise a set of defined protocols to cooperatively handleuser interaction with the netpage system.

Each protocol is illustrated by way of a sequence diagram in which thehorizontal dimension is used to represent message flow and the verticaldimension is used to represent time. Each entity is represented by arectangle containing the name of the entity and a vertical columnrepresenting the lifeline of the entity. During the time an entityexists, the lifeline is shown as a dashed line. During the time anentity is active, the lifeline is shown as a double line. Because theprotocols considered here do not create or destroy entities, lifelinesare generally cut short as soon as an entity ceases to participate in aprotocol.

5.1 Subscription Delivery Protocol

A preferred embodiment of a subscription delivery protocol is shown inFIG. 43.

A large number of users may subscribe to a periodical publication. Eachuser's edition may be laid out differently, but many users' editionswill share common content such as text objects and image objects. Thesubscription delivery protocol therefore delivers document structures toindividual printers via pointcast, but delivers shared content objectsvia multicast.

The application (i.e. publisher) first obtains a document ID 51 for eachdocument from an ID server 12. It then sends each document structure,including its document ID and page descriptions, to the page server 10responsible for the document's newly allocated ID. It includes its ownapplication ID 64, the subscriber's alias ID 65, and the relevant set ofmulticast channel names. It signs the message using its privatesignature key.

The page server uses the application ID and alias ID to obtain from theregistration server the corresponding user ID 60, the user's selectedprinter ID 62 (which may be explicitly selected for the application, ormay be the user's default printer), and the application's certificate.

The application's certificate allows the page server to verify themessage signature. The page server's request to the registration serverfails if the application ID and alias ID don't together identify asubscription 808.

The page server then allocates document and page instance IDs andforwards the page descriptions, including page IDs 50, to the printer.It includes the relevant set of multicast channel names for the printerto listen to.

It then returns the newly allocated page IDs to the application forfuture reference.

Once the application has distributed all of the document structures tothe subscribers' selected printers via the relevant page servers, itmulticasts the various subsets of the shared objects on the previouslyselected multicast channels. Both page servers and printers monitor theappropriate multicast channels and receive their required contentobjects. They are then able to populate the previously pointcastdocument structures. This allows the page servers to add completedocuments to their databases, and it allows the printers to print thedocuments.

5.2 Hyperlink Activation Protocol

A preferred embodiment of a hyperlink activation protocol is shown inFIG. 45.

When a user clicks on a netpage with a netpage pen, the pen communicatesthe click to the nearest netpage printer 601. The click identifies thepage and a location on the page. The printer already knows the ID 61 ofthe pen from the pen connection protocol.

The printer determines, via the DNS, the network address of the pageserver 10 a handling the particular page ID 50. The address may alreadybe in its cache if the user has recently interacted with the same page.The printer then forwards the pen ID, its own printer ID 62, the page IDand click location to the page server.

The page server loads the page description 5 identified by the page IDand determines which input element's zone 58, if any, the click lies in.Assuming the relevant input element is a hyperlink element 844, the pageserver then obtains the associated application ID 64 and link ID 54, anddetermines, via the DNS, the network address of the application serverhosting the application 71.

The page server uses the pen ID 61 to obtain the corresponding user ID60 from the registration server 11, and then allocates a globally uniquehyperlink request ID 52 and builds a hyperlink request 934. Thehyperlink request class diagram is shown in FIG. 44. The hyperlinkrequest records the IDs of the requesting user and printer, andidentifies the clicked hyperlink instance 862. The page server thensends its own server ID 53, the hyperlink request ID, and the link ID tothe application.

The application produces a response document according toapplication-specific logic, and obtains a document ID 51 from an IDserver 12. It then sends the document to the page server 10 bresponsible for the document's newly allocated ID, together with therequesting page server's ID and the hyperlink request ID.

The second page server sends the hyperlink request ID and application IDto the first page server to obtain the corresponding user ID and printerID 62. The first page server rejects the request if the hyperlinkrequest has expired or is for a different application.

The second page server allocates document instance and page IDs 50,returns the newly allocated page IDs to the application, adds thecomplete document to its own database, and finally sends the pagedescriptions to the requesting printer.

The hyperlink instance may include a meaningful transaction ID 55, inwhich case the first page server includes the transaction ID in themessage sent to the application. This allows the application toestablish a transaction-specific context for the hyperlink activation.

If the hyperlink requires a user alias, i.e. its “alias required”attribute is set, then the first page server sends both the pen ID 61and the hyperlink's application ID 64 to the registration server 11 toobtain not just the user ID corresponding to the pen ID but also thealias ID 65 corresponding to the application ID and the user ID. Itincludes the alias ID in the message sent to the application, allowingthe application to establish a user-specific context for the hyperlinkactivation.

5.3 Handwriting Recognition Protocol

When a user draws a stroke on a netpage with a netpage pen, the pencommunicates the stroke to the nearest netpage printer. The strokeidentifies the page and a path on the page.

The printer forwards the pen ID 61, its own printer ID 62, the page ID50 and stroke path to the page server 10 in the usual way.

The page server loads the page description 5 identified by the page IDand determines which input element's zone 58, if any, the strokeintersects. Assuming the relevant input element is a text field 878, thepage server appends the stroke to the text field's digital ink.

After a period of inactivity in the zone of the text field, the pageserver sends the pen ID and the pending strokes to the registrationserver 11 for interpretation. The registration server identifies theuser corresponding to the pen, and uses the user's accumulatedhandwriting model 822 to interpret the strokes as handwritten text. Onceit has converted the strokes to text, the registration server returnsthe text to the requesting page server. The page server appends the textto the text value of the text field.

5.4 Signature Verification Protocol

Assuming the input element whose zone the stroke intersects is asignature field 880, the page server 10 appends the stroke to thesignature field's digital ink.

After a period of inactivity in the zone of the signature field, thepage server sends the pen ID 61 and the pending strokes to theregistration server 11 for verification. It also sends the applicationID 64 associated with the form of which the signature field is part, aswell as the form ID 56 and the current data content of the form. Theregistration server identifies the user corresponding to the pen, anduses the user's dynamic signature biometric 818 to verify the strokes asthe user's signature. Once it has verified the signature, theregistration server uses the application ID 64 and user ID 60 toidentify the user's application-specific private signature key. It thenuses the key to generate a digital signature of the form data, andreturns the digital signature to the requesting page server. The pageserver assigns the digital signature to the signature field and sets theassociated form's status to frozen.

The digital signature includes the alias ID 65 of the correspondinguser. This allows a single form to capture multiple users' signatures.

5.5 Form Submission Protocol

A preferred embodiment of a form submission protocol is shown in FIG.46.

Form submission occurs via a form hyperlink activation. It thus followsthe protocol defined in Section 5.2, with some form-specific additions.

In the case of a form hyperlink, the hyperlink activation message sentby the page server 10 to the application 71 also contains the form ID 56and the current data content of the form. If the form contains anysignature fields, then the application verifies each one by extractingthe alias ID 65 associated with the corresponding digital signature andobtaining the corresponding certificate from the registration server 11.

5.6 Commission Payment Protocol

A preferred embodiment of a commission payment protocol is shown in FIG.47.

In an e-commerce environment, fees and commissions may be payable froman application provider to a publisher on click-throughs, transactionsand sales. Commissions on fees and commissions on commissions may alsobe payable from the publisher to the provider of the printer.

The hyperlink request ID 52 is used to route a fee or commission creditfrom the target application provider 70 a (e.g. merchant) to the sourceapplication provider 70 b (i.e. publisher), and from the sourceapplication provider 70 b to the printer provider 72.

The target application receives the hyperlink request ID from the pageserver 10 when the hyperlink is first activated, as described in Section5.2. When the target application needs to credit the source applicationprovider, it sends the application provider credit to the original pageserver together with the hyperlink request ID. The page server uses thehyperlink request ID to identify the source application, and sends thecredit on to the relevant registration server 11 together with thesource application ID 64, its own server ID 53, and the hyperlinkrequest ID. The registration server credits the correspondingapplication provider's account 827. It also notifies the applicationprovider.

If the application provider needs to credit the printer provider, itsends the printer provider credit to the original page server togetherwith the hyperlink request ID. The page server uses the hyperlinkrequest ID to identify the printer, and sends the credit on to therelevant registration server together with the printer ID. Theregistration server credits the corresponding printer provider account814.

The source application provider is optionally notified of the identityof the target application provider, and the printer provider of theidentity of the source application provider.

6. Netpage Pen Description

6.1 Pen Mechanics

Referring to FIGS. 8 and 9, the pen, generally designated by referencenumeral 101, includes a housing 102 in the form of a plastics mouldinghaving walls 103 defining an interior space 104 for mounting the pencomponents. The pen top 105 is in operation rotatably mounted at one end106 of the housing 102. A semi-transparent cover 107 is secured to theopposite end 108 of the housing 102. The cover 107 is also of mouldedplastics, and is formed from semi-transparent material in order toenable the user to view the status of the LED mounted within the housing102. The cover 107 includes a main part 109 which substantiallysurrounds the end 108 of the housing 102 and a projecting portion 110which projects back from the main part 109 and fits within acorresponding slot 111 formed in the walls 103 of the housing 102. Aradio antenna 112 is mounted behind the projecting portion 110, withinthe housing 102. Screw threads 113 surrounding an aperture 113A on thecover 107 are arranged to receive a metal end piece 114, includingcorresponding screw threads 115. The metal end piece 114 is removable toenable ink cartridge replacement.

Also mounted within the cover 107 is a tri-color status LED 116 on aflex PCB 117. The antenna 112 is also mounted on the flex PCB 117. Thestatus LED 116 is mounted at the top of the pen 101 for good all-aroundvisibility.

The pen can operate both as a normal marking ink pen and as anon-marking stylus. An ink pen cartridge 118 with nib 119 and a stylus120 with stylus nib 121 are mounted side by side within the housing 102.Either the ink cartridge nib 119 or the stylus nib 121 can be broughtforward through open end 122 of the metal end piece 114, by rotation ofthe pen top 105. Respective slider blocks 123 and 124 are mounted to theink cartridge 118 and stylus 120, respectively. A rotatable cam barrel125 is secured to the pen top 105 in operation and arranged to rotatetherewith. The cam barrel 125 includes a cam 126 in the form of a slotwithin the walls 181 of the cam barrel. Cam followers 127 and 128projecting from slider blocks 123 and 124 fit within the cam slot 126.On rotation of the cam barrel 125, the slider blocks 123 or 124 moverelative to each other to project either the pen nib 119 or stylus nib121 out through the hole 122 in the metal end piece 114. The pen 101 hasthree states of operation. By turning the top 105 through 90° steps, thethree states are:

-   -   Stylus 120 nib 121 out;    -   Ink cartridge 118 nib 119 out; and    -   Neither ink cartridge 118 nib 119 out nor stylus 120 nib 121        out.

A second flex PCB 129, is mounted on an electronics chassis 130 whichsits within the housing 102. The second flex PCB 129 mounts an infraredLED 131 for providing infrared radiation for projection onto thesurface. An image sensor 132 is provided mounted on the second flex PCB129 for receiving reflected radiation from the surface. The second flexPCB 129 also mounts a radio frequency chip 133, which includes an RFtransmitter and RF receiver, and a controller chip 134 for controllingoperation of the pen 101. An optics block 135 (formed from moulded clearplastics) sits within the cover 107 and projects an infrared beam ontothe surface and receives images onto the image sensor 132. Power supplywires 136 connect the components on the second flex PCB 129 to batterycontacts 137 which are mounted within the cam barrel 125. A terminal 138connects to the battery contacts 137 and the cam barrel 125. A threevolt rechargeable battery 139 sits within the cam barrel 125 in contactwith the battery contacts. An induction charging coil 140 is mountedabout the second flex PCB 129 to enable recharging of the battery 139via induction. The second flex PCB 129 also mounts an infrared LED 143and infrared photodiode 144 for detecting displacement in the cam barrel125 when either the stylus 120 or the ink cartridge 118 is used forwriting, in order to enable a determination of the force being appliedto the surface by the pen nib 119 or stylus nib 121. The IR photodiode144 detects light from the IR LED 143 via reflectors (not shown) mountedon the slider blocks 123 and 124.

Rubber grip pads 141 and 142 are provided towards the end 108 of thehousing 102 to assist gripping the pen 101, and top 105 also includes aclip 142 for clipping the pen 101 to a pocket.

6.2 Pen Controller

The pen 101 is arranged to determine the position of its nib (stylus nib121 or ink cartridge nib 119) by imaging, in the infrared spectrum, anarea of the surface in the vicinity of the nib. It records the locationdata from the nearest location tag, and is arranged to calculate thedistance of the nib 121 or 119 from the location tab utilising optics135 and controller integrated circuit 134. The controller integratedcircuit 134 calculates the orientation of the pen and the nib-to-tagdistance from the perspective distortion observed on the imaged tag.

Utilising the RF integrated circuit 133 and antenna 112 the pen 101 cantransmit the digital ink data (which is encrypted for security andpackaged for efficient transmission) to the computing system.

When the pen is in range of a receiver, the digital ink data istransmitted as it is formed. When the pen 101 moves out of range,digital ink data is buffered within the pen 101 (the pen 101 circuitryincludes a buffer arranged to store digital ink data for approximately12 minutes of the pen motion on the surface) and can be transmittedlater.

The controller integrated circuit 134 is mounted on the second flex PCB129 in the pen 101. FIG. 10 is a block diagram illustrating in moredetail the architecture of the controller integrated circuit 134. FIG.10 also shows representations of the RF integrated circuit 133, theimage sensor 132, the tri-color status LED 116, the IR illumination LED131, the IR force sensor LED 143, and the force sensor photodiode 144.

The pen controller integrated circuit 134 includes a controllingprocessor 145. Bus 146 enables the exchange of data between componentsof the controller integrated circuit 134. Flash memory 147 and a 512 KBDRAM 148 are also included. An analog-to-digital converter 149 isarranged to convert the analog signal from the force sensor photodiode144 to a digital signal.

An image sensor interface 152 interfaces with the image sensor 132. Atransceiver controller 153 and base band circuit 154 are also includedto interface with the RF integrated circuit 133 which includes an RFcircuit 155 and RF resonators and inductors 156 connected to the antenna112.

The controlling processor 145 captures and decodes location data fromtags from the surface via the image sensor 132, monitors the forcesensor photodiode 144, controls the LEDs 116, 131 and 143, and handlesshort-range radio communication via the radio transceiver 153. It is amedium-performance (˜40 MHz) general-purpose RISC processor.

The processor 145, digital transceiver components (transceivercontroller 153 and baseband circuit 154), image sensor interface 152,flash memory 147 and 512 KB DRAM 148 are integrated in a singlecontroller ASIC. Analog RF components (RF circuit 155 and RF resonatorsand inductors 156) are provided in the separate RF integrated circuit.

The image sensor is a 215×215 pixel CCD (such a sensor is produced byMatsushita Electronic Corporation, and is described in a paper byItakura, K T Nobusada, N Okusenya, R Nagayoshi, and M Ozaki, “A 1 mm 50k-Pixel IT CCD Image Sensor for Miniature Camera System”, IEEETransactions on Electronic Devices, Volt 47, number 1, January 2000,which is incorporated herein by reference) with an IR filter.

The controller ASIC 134 enters a quiescent state after a period ofinactivity when the pen 101 is not in contact with a surface. Itincorporates a dedicated circuit 150 which monitors the force sensorphotodiode 144 and wakes up the controller 134 via the power manager 151on a pen-down event.

The radio transceiver communicates in the unlicensed 900 MHz bandnormally used by cordless telephones, or alternatively in the unlicensed2.4 GHz industrial, scientific and medical (ISM) band, and usesfrequency hopping and collision detection to provide interference-freecommunication.

In an alternative embodiment, the pen incorporates an Infrared DataAssociation (IrDA) interface for short-range communication with a basestation or netpage printer.

In a further embodiment, the pen 101 includes a pair of orthogonalaccelerometers mounted in the normal plane of the pen 101 axis. Theaccelerometers 190 are shown in FIGS. 9 and 10 in ghost outline.

The provision of the accelerometers enables this embodiment of the pen101 to sense motion without reference to surface location tags, allowingthe location tags to be sampled at a lower rate. Each location tag IDcan then identify an object of interest rather than a position on thesurface. For example, if the object is a user interface input element(e.g. a command button), then the tag ID of each location tag within thearea of the input element can directly identify the input element.

The acceleration measured by the accelerometers in each of the x and ydirections is integrated with respect to time to produce aninstantaneous velocity and position.

Since the starting position of the stroke is not known, only relativepositions within a stroke are calculated. Although position integrationaccumulates errors in the sensed acceleration, accelerometers typicallyhave high resolution, and the time duration of a stroke, over whicherrors accumulate, is short.

7. Netpage Printer Description

7.1 Printer Mechanics

The vertically-mounted netpage wallprinter 601 is shown fully assembledin FIG. 11.

It prints netpages on Letter/A4 sized media using duplexed 8½″ Memjet™print engines 602 and 603, as shown in FIGS. 12 and 12 a. It uses astraight paper path with the paper 604 passing through the duplexedprint engines 602 and 603 which print both sides of a sheetsimultaneously, in full color and with full bleed.

An integral binding assembly 605 applies a strip of glue along one edgeof each printed sheet, allowing it to adhere to the previous sheet whenpressed against it. This creates a final bound document 618 which canrange in thickness from one sheet to several hundred sheets.

The replaceable ink cartridge 627, shown in FIG. 13 coupled with theduplexed print engines, has bladders or chambers for storing fixative,adhesive, and cyan, magenta, yellow, black and infrared inks. Thecartridge also contains a micro air filter in a base molding. The microair filter interfaces with an air pump 638 inside the printer via a hose639. This provides filtered air to the printheads to prevent ingress ofmicro particles into the Memjet™ printheads 350 which might otherwiseclog the printhead nozzles. By incorporating the air filter within thecartridge, the operational life of the filter is effectively linked tothe life of the cartridge. The ink cartridge is a fully recyclableproduct with a capacity for printing and gluing 3000 pages (1500sheets).

Referring to FIG. 12, the motorized media pick-up roller assembly 626pushes the top sheet directly from the media tray past a paper sensor onthe first print engine 602 into the duplexed Memjet™ printhead assembly.The two Memjet™ print engines 602 and 603 are mounted in an opposingin-line sequential configuration along the straight paper path. Thepaper 604 is drawn into the first print engine 602 by integral, poweredpick-up rollers 626. The position and size of the paper 604 is sensedand full bleed printing commences. Fixative is printed simultaneously toaid drying in the shortest possible time.

The paper exits the first Memjet™ print engine 602 through a set ofpowered exit spike wheels (aligned along the straight paper path), whichact against a rubberized roller. These spike wheels contact the ‘wet’printed surface and continue to feed the sheet 604 into the secondMemjet™ print engine 603.

Referring to FIGS. 12 and 12 a, the paper 604 passes from the duplexedprint engines 602 and 603 into the binder assembly 605. The printed pagepasses between a powered spike wheel axle 670 with a fibrous supportroller and another movable axle with spike wheels and a momentary actionglue wheel. The movable axle/glue assembly 673 is mounted to a metalsupport bracket and it is transported forward to interface with thepowered axle 670 via gears by action of a camshaft. A separate motorpowers this camshaft.

The glue wheel assembly 673 consists of a partially hollow axle 679 witha rotating coupling for the glue supply hose 641 from the ink cartridge627. This axle 679 connects to a glue wheel, which absorbs adhesive bycapillary action through radial holes. A molded housing 682 surroundsthe glue wheel, with an opening at the front. Pivoting side moldings andsprung outer doors are attached to the metal bracket and hinge outsideways when the rest of the assembly 673 is thrust forward. Thisaction exposes the glue wheel through the front of the molded housing682. Tension springs close the assembly and effectively cap the gluewheel during periods of inactivity.

As the sheet 604 passes into the glue wheel assembly 673, adhesive isapplied to one vertical edge on the front side (apart from the firstsheet of a document) as it is transported down into the binding assembly605.

7.2 Printer Controller Architecture

The netpage printer controller consists of a controlling processor 750,a factory-installed or field-installed network interface module 625, aradio transceiver (transceiver controller 753, baseband circuit 754, RFcircuit 755, and RF resonators and inductors 756), dual raster imageprocessor (RIP) DSPs 757, duplexed print engine controllers 760 a and760 b, flash memory 658, and 64 MB of DRAM 657, as illustrated in FIG.14. The controlling processor handles communication with the network 19and with local wireless netpage pens 101, senses the help button 617,controls the user interface LEDs 613-616, and feeds and synchronizes theRIP DSPs 757 and print engine controllers 760. It consists of amedium-performance general-purpose microprocessor. The controllingprocessor 750 communicates with the print engine controllers 760 via ahigh-speed serial bus 659.

The RIP DSPs rasterize and compress page descriptions to the netpageprinter's compressed page format. Each print engine controller expands,dithers and prints page images to its associated Memjet™ printhead 350in real time (i.e. at over 30 pages per minute). The duplexed printengine controllers print both sides of a sheet simultaneously.

The master print engine controller 760 a controls the paper transportand monitors ink usage in conjunction with the master QA integratedcircuit 665 and the ink cartridge QA integrated circuit 761.

The printer controller's flash memory 658 holds the software for boththe processor 750 and the DSPs 757, as well as configuration data. Thisis copied to main memory 657 at boot time.

The processor 750, DSPs 757, and digital transceiver components(transceiver controller 753 and baseband circuit 754) are integrated ina single controller ASIC 656. Analog RF components (RF circuit 755 andRF resonators and inductors 756) are provided in a separate RFintegrated circuit 762. The network interface module 625 is separate,since netpage printers allow the network connection to befactory-selected or field-selected. Flash memory 658 and the 2×256 Mbit(64 MB) DRAM 657 is also off-integrated circuit. The print enginecontrollers 760 are provided in separate ASICs.

A variety of network interface modules 625 are provided, each providinga netpage network interface 751 and optionally a local computer ornetwork interface 752. Netpage network Internet interfaces include POTSmodems, Hybrid Fiber-Coax (HFC) cable modems, ISDN modems, DSL modems,satellite transceivers, current and next-generation cellular telephonetransceivers, and wireless local loop (WLL) transceivers. Localinterfaces include IEEE 1284 (parallel port), 10 Base-T and 100 Base-TEthernet, USB and USB 2.0, IEEE 1394 (Firewire), and various emerginghome networking interfaces. If an Internet connection is available onthe local network, then the local network interface can be used as thenetpage network interface.

The radio transceiver 753 communicates in the unlicensed 900 MHz bandnormally used by cordless telephones, or alternatively in the unlicensed2.4 GHz industrial, scientific and medical (ISM) band, and usesfrequency hopping and collision detection to provide interference-freecommunication.

The printer controller optionally incorporates an Infrared DataAssociation (IrDA) interface for receiving data “squirted” from devicessuch as netpage cameras. In an alternative embodiment, the printer usesthe IrDA interface for short-range communication with suitablyconfigured netpage pens.

7.2.1 Rasterization and Printing

Once the main processor 750 has received and verified the document'spage layouts and page objects, it runs the appropriate RIP software onthe DSPs 757.

The DSPs 757 rasterize each page description and compress the rasterizedpage image. The main processor stores each compressed page image inmemory. The simplest way to load-balance multiple DSPs is to let eachDSP rasterize a separate page. The DSPs can always be kept busy since anarbitrary number of rasterized pages can, in general, be stored inmemory. This strategy only leads to potentially poor DSP utilizationwhen rasterizing short documents.

Watermark regions in the page description are rasterized to acontone-resolution bi-level bitmap which is losslessly compressed tonegligible size and which forms part of the compressed page image.

The infrared (IR) layer of the printed page contains coded netpage tagsat a density of about six per inch. Each tag encodes the page ID, tagID, and control bits, and the data content of each tag is generatedduring rasterization and stored in the compressed page image.

The main processor 750 passes back-to-back page images to the duplexedprint engine controllers 760. Each print engine controller 760 storesthe compressed page image in its local memory, and starts the pageexpansion and printing pipeline. Page expansion and printing ispipelined because it is impractical to store an entire 114 MB bi-levelCMYK+IR page image in memory.

7.2.2 Print Engine Controller

The page expansion and printing pipeline of the print engine controller760 consists of a high speed IEEE 1394 serial interface 659, a standardJPEG decoder 763, a standard Group 4 Fax decoder 764, a customhalftoner/compositor unit 765, a custom tag encoder 766, a lineloader/formatter unit 767, and a custom interface 768 to the Memjet™printhead 350.

The print engine controller 360 operates in a double buffered manner.While one page is loaded into DRAM 769 via the high speed serialinterface 659, the previously loaded page is read from DRAM 769 andpassed through the print engine controller pipeline. Once the page hasfinished printing, the page just loaded is printed while another page isloaded.

The first stage of the pipeline expands (at 763) the JPEG-compressedcontone CMYK layer, expands (at 764) the Group 4 Fax-compressed bi-levelblack layer, and renders (at 766) the bi-level netpage tag layeraccording to the tag format defined in section 1.2, all in parallel. Thesecond stage dithers (at 765) the contone CMYK layer and composites (at765) the bi-level black layer over the resulting bi-level CMYK layer.The resultant bi-level CMYK+IR dot data is buffered and formatted (at767) for printing on the Memjet™ printhead 350 via a set of linebuffers. Most of these line buffers are stored in the off-integratedcircuit DRAM. The final stage prints the six channels of bi-level dotdata (including fixative) to the Memjet™ printhead 350 via the printheadinterface 768.

When several print engine controllers 760 are used in unison, such as ina duplexed configuration, they are synchronized via a shared line syncsignal 770. Only one print engine 760, selected via the externalmaster/slave pin 771, generates the line sync signal 770 onto the sharedline.

The print engine controller 760 contains a low-speed processor 772 forsynchronizing the page expansion and rendering pipeline, configuring theprinthead 350 via a low-speed serial bus 773, and controlling thestepper motors 675, 676.

In the 8½″ versions of the netpage printer, the two print engines eachprints 30 Letter pages per minute along the long dimension of the page(11″), giving a line rate of 8.8 kHz at 1600 dpi. In the 12″ versions ofthe netpage printer, the two print engines each prints 45 Letter pagesper minute along the short dimension of the page (8½″), giving a linerate of 10.2 kHz. These line rates are well within the operatingfrequency of the Memjet™ printhead, which in the current design exceeds30 kHz.

8 Netpage Tags

8.1 Tag Tiling

8.1.1 Planar Surface Tag Tiling

In order to support “single-click” interaction with a tagged region viaa sensing device, the sensing device must be able to see at least oneentire tag 4 in its field of view no matter where in the region or atwhat orientation it is positioned. The required diameter of the field ofview of the sensing device is therefore a function of the size andspacing of the tags 4.

In the case where the tag shape is circular, such as the preferred tag 4described earlier, the minimum diameter m of the sensor field of view isobtained when the tags 500, of diameter k, are tiled on an equilateraltriangular grid, as shown in FIG. 52 and defined in EQ 1. This isachieved when the center-to-center tag spacing is the same as the tagdiameter k.

With a tag diameter k of 256 dots (˜4 mm at 1600 dpi), m is therefore552 dots (˜8.8 mm). With a quiet area of 16 dots, i.e. an effective tagdiameter k of 272 dots (˜4.3 mm), m increases to 587 dots (˜9.3 mm).

When the tags 4 are moved a distance s apart, where s is at least aslarge as k, then the minimum field of view is given by EQ 2.

When no overlap is desired in the horizontal direction betweensuccessive lines of tags 500, for example to make tag rendering easier,the tags must be moved apart by a minimum amount given by EQ 3. For a256-dot diameter tag, u is therefore 40 dots (˜0.6 mm at 1600 dpi).Since this exceeds the quiet area required for the tag, the quiet areacan be ignored if tag lines are rendered to not overlap.

Setting s=k+u in EQ 2 gives EQ 4. For a 256-dot diameter tag, s istherefore 296 dots (˜4.7 mm at 1600 dpi), and m is 598 dots (˜9.5 mm).

8.1.2 Spherical Surface Tag Tiling

A regular icosahedron is often used as the basis for generating analmost regular triangular tiling of a sphere. A regular icosahedron,such as icosahedron 526 in FIG. 53, is composed of twenty equal-sizedequilateral triangular faces 528 sharing thirty edges 530 and twelvevertices 532, with five of the edges 530 meeting at each of the vertices532.

To achieve the required tiling, the icosahedron 526 is inscribed in atarget sphere, and each triangle 528 of the icosahedron 526 issubdivided into an equal number of equal-sized equilateral subdivisiontriangles to yield the desired total number of triangles. If each edge530 of the icosahedron is divided into v equal intervals, defining a setof v−1 points along each edge, and each pair of corresponding pointsalong any two adjacent edges is joined by a line parallel to the othershared adjacent edge, the lines so drawn intersect at the vertices ofthe desired equal-sized and equilateral subdivision triangles, resultingin the creation of v² triangles per triangular face 528 of theicosahedron 526, or 20v² triangles in all. Of the resulting 10v²+2vertices, five triangular faces meet at each of the twelve originalvertices of the icosahedron 526, and six triangular faces meet at theeach of the remaining vertices. The twelve original vertices 532 alreadylie on the sphere, while the remaining vertices lie inside the sphere.Each created vertex is therefore centrally projected onto the sphere,giving the desired tiling.

A sphere approximated by a regular polyhedron in this way is referred toas a geodesic, and the parameter v is referred to as the frequency ofthe geodesic. FIG. 54 shows an icosahedral geodesic 534 with v=3, i.e.with 180 faces 528.

The closer a subdivision triangle is to the center of a face of theicosahedron 526, the further it is from the surface of the sphere, andhence the larger it is when projected onto the sphere. To minimisevariation in the size of projected subdivision triangles, subdivisonvertices can systematically be displaced prior to projection (Tegmark,M., “An Icosahedron-Based Method for Pixelizing the Celestial Sphere”,ApJ Letters, 470, L81, Oct. 14, 1996). If v=1 then no vertices arecreated and the angle subtended by a triangular face at a vertex remains60°. As v increases, however, the surface defined by the five triangularfaces surrounding each original vertex becomes increasingly flat, andthe vertex angle of each triangular face converges on 72° (i.e. 360°/5).This defines the worst case for a tag tiling of a sphere. In a 72°isosceles triangle the base length is 1.18 times the length of the twosides. The maximum tag spacing s for the purposes of calculating thesensor field of view is therefore close to 1.18 k. With a tag diameterof 256 dots and a quiet area of 16 dots, i.e. an effective tag diameterk of 272 dots (˜4.3 mm), m is therefore 643 dots (˜10.2 mm) according toEQ 2.

The angle subtended by each edge of an icosahedron at the center of thecircumscribing sphere is given by EQ 5

For a sphere of radius r the arc length of each centrally projected edgeis rθ. Given a tag diameter of K in the same units as r, the number oftags n required to cover the sphere is given by EQ 6.

For a given n, r is limited by EQ 7.

If n is limited to 2¹⁶, to allow the use of a 16-bit tag ID withoutrequiring multiple regions to cover the sphere, and K is taken to be 4.3mm as above, then r is limited to ˜310 mm.

A typical globe has a radius of 160 mm. Its projected arc length of ˜177mm fits 41 evenly spaced tags with negligible additional spacing. Such aglobe uses 16812 tags in total.

8.1.3 Arbitrary Curved Surface Tag Tiling

A triangle mesh can approximate a surface of arbitrary topography andtopology without introducing discontinuities or singularities, with thelocal scale of the mesh being dictated by the local curvature of thesurface and an error bound. Assuming the existence of a triangle meshfor a particular surface, an effective non-regular tiling of tags can beproduced as long as each mesh triangle respects a minimum vertex angleand a minimum edge length. A tiling is considered effective with respectto a particular sensing device if the field of view of the sensingdevice is guaranteed to include at least one complete tag at anyposition of the sensing device on the surface.

The tiling procedure starts by placing a tag at each vertex of the mesh,so the minimum edge length is the same as the tag diameter k. The tilingprocedure proceeds by inserting a tag at the midpoint of any edge whoselength exceeds a maximum tag separation s. As illustrated in FIG. 9, themaximum tag spacing s is calculated so that if two adjacent tags 4 a and4 b are a distance s+ε apart, then there is room for another tag 4 cbetween them, i.e. EQ 8.

However, if the vertex angle between two edges of length s+ε is lessthan 60°, then the inserted tags will overlap.

To prevent inserted tags from overlapping, a minimum tag separation t isintroduced, where t≧k. The minimum vertex angle α then becomes afunction of k and t, as shown in EQ 9.

Clearly, when t=k, β is constrained to be 60°, i.e. the mesh isconstrained to be equilateral. But as illustrated in FIG. 56, when t>k,β can be less than 60° without inserted tags overlapping.

The maximum tag separation s must be based on the new minimum tagseparation t, in accordance with EQ 10.

When considering a particular mesh triangle, there are four distinct taginsertion scenarios. By assuming that the minimum vertex angle is noless than 30° (i.e. half of 60°), it can be shown that whenever a meshtriangle has at least one edge less than or equal to s in length, theremaining two edges are less than 2 s in length. In practice the minimumvertex angle is typically at least 45°.

In the first scenario (FIG. 57) no edges of a triangle 546 exceed s inlength, so the tagging of the triangle is already complete.

In the second scenario (FIG. 58) one edge 548 of a triangle 550 exceedss in length. A tag 552 is inserted at the midpoint of the edge 548 tocomplete the tagging of the triangle 550.

In the third scenario (FIG. 59) two edges 554, 556 of a triangle 558exceed s in length. Tags 560, 562 are inserted at the midpoint of eachof the two long edges 554, 556 and this may complete the tagging of thetriangle 558. Centers of the two inserted tags 560, 562 together withthe two vertices 564, 566 of the short edge 568 of the original triangle558 form a trapezoid. If either diagonal of the trapezoid exceeds s inlength then a final tag 570 is inserted at the center of the trapezoidto complete the tagging of the triangle.

In the fourth scenario (FIG. 60) all three edges 572 of a triangle 573exceed s in length. A tagged vertex 574 is inserted at the midpoint ofeach edge 572 and the three new vertices 574 are joined by edges 576.The tagging procedure is then recursively applied to each of the fourresultant triangles 577, 578, 579 and 580. Note that the new trianglesrespect the minimum vertex angle because they have the same shape as theoriginal triangle 573.

The tag tiling variables are summarized in Table 4.

TABLE 4 Tag tiling variables variable Meaning β minimum vertex angle ktag diameter m minimum diameter of sensor field of view on surface smaximum center-to-center tag spacing t minimum center-to-center tagspacing8.2 Tag Sensing8.2.1 Pen Orientation

To allow a pen-like sensing device to be used as a comfortable writinginstrument, a range of pen orientations must be supported. Since the pennib is constrained to be in contact with the surface, the orientation ofthe pen can be characterized by the yaw (z rotation), pitch (x rotation)and roll (y rotation) of the pen, as illustrated in FIG. 61. While theyaw of the pen must be unconstrained, it is reasonable to constrain thepitch and roll of the pen as well as the overall tilt of the penresulting from the combination of pitch and roll.

Yaw is conventionally applied after pitch, such that, for example, inthe case of a pen device it would define a twist about the physical axisrather than a direction in the surface plane. In a pen with a markingnib, however, the image sensor is mounted off the axis of the pen andthe pen's image sensing ability (and hence its yaw sensing ability) istherefore constrained unless the pen is held almost vertically, asdiscussed below. Yaw is therefore applied before pitch, allowing thefull yaw range to be specified by rotating the pen relative to thesurface while keeping pitch and roll constant.

Pitch and roll are conventionally defined as y and x rotations,respectively. Here they are defined as x and y rotations, respectively,because they are defined with respect to the x-y coordinate system ofthe surface, where the y axis is the natural longitudinal axis and the xaxis is the natural lateral axis when viewed by a user. In aright-handed 3D coordinate system, roll is conventionally defined aspositive when anticlockwise, while pitch and yaw are conventionallydefined as positive when clockwise. Here all rotations are defined aspositive when anticlockwise.

The pen's overall tilt (θ) is related to its pitch (φ) and the roll (ψ)in accordance with EQ 11.

The pen's tilt affects the scale at which surface features are imaged atdifferent points in the field of view, and therefore affects theresolution of the image sensor. Since it is impractical to sense thearea directly under the pen nib, the pen's tilt also affects thedistance from the nib to the center of the imaged area. This distancemust be known to allow a precise nib position to be derived from theposition determined from the tag.

8.2.2 Image Sensing

The field of view can be modeled as a cone defined by a solid half-angleα (giving an angular field of view of 2α), and an apex height of D abovethe surface when the optical axis is vertical. Although the image sensoris typically rectangular, only the largest elliptical subarea of theimage sensor is relevant to guaranteeing that a sufficiently large partof the surface is imaged, as quantified earlier.

The intersection of the field of view cone with the surface defines anelliptical window on the surface. This window is circular when theoptical axis is vertical.

FIG. 62 illustrates the geometric relationship, for a givenpitch-related tilt θ of the pen's optical axis, between the pen's nib(point A), the pen's optical axis (CE), and the field of view window(FH). The tilt is defined to be clockwise positive from the vertical.The equations which follow apply to both positive and negative tilt.

When the pen is not tilted, the window diameter (i.e. |BD|) is given byEQ 12.

If, when the pen is not tilted, the distance from the nib to edge of thewindow (i.e. |AB|) is T, then the distance S from the nib to the centerof the window (i.e. |AC|) is given by EQ 13.

When the pen is tilted by θ, the distance from the viewpoint to thesurface along the optical axis is reduced to d (i.e. |GE|), given by EQ14.

The width of the window (i.e. |FH|) is then given by EQ 15.

D and α must be chosen so that an adequately large area is imagedthroughout the supported tilt range. The required minimum diameter m ofthe area is given by EQ 4, while the width of the actual imaged area isgiven by EQ 15. This then gives EQ 16.

Once D and α are determined, an image sensor resolution must be chosenso that the imaged area is adequately sampled, i.e. that the maximumfeature frequency is sampled at its Nyquist rate or higher.

When imaged, the scale of the surface decreases with increasing distancefrom the viewpoint and with increasing inclination relative to theviewing ray. Both factors have maximum effect at point F for positivetilt and point H for negative tilt, i.e. at the point in the windowfurthest from the viewpoint. Note that references to F in the followingdiscussion apply to H when the tilt is negative.

The distance of point F from the viewpoint (i.e. |EF|) is given by EQ17.

Scaling due to the inclination of the surface relative to the viewingray through F (EF) is given by EQ 18.

If the surface feature frequency is f, then the angular surface featurefrequency ω at F (i.e. with respect to the field of view) due to bothfactors is given by EQ 19.

When there is no object plane tilt (i.e. θ=0), this reduces to EQ 20.

The image sensor is, by definition, required to image at least theentire angular field of view. Since the pixel density of the imagesensor is uniform, it must image the entire field of view at maximumfrequency. Given an angular field of view in image space of 2α′, animage sensor tilt (i.e. image plane tilt) with respect to the opticalaxis of θ′, and a sampling rate of n (where n≧2 according to Nyquist'stheorem), the minimum image sensor resolution q is given by EQ 21 and EQ22.

The cos-squared term in the numerator in EQ 22 results from the samereasoning as the cos-squared term in the denominator in EQ 19.

When there is no image plane tilt (i.e. θ′=0), and the image space andobject space angular fields of view are equal (i.e. α′=α), this reducesto EQ 23 and EQ 24.

When there is no object plane tilt (i.e. θ=0) this reduces further to EQ25.

When the image plane tilt and the object plane tilt are equal (i.e.θ′=0), and the image space and object space angular fields of view areequal (i.e. α′=α), EQ 22 reduces to EQ 26.

Matching the image plane tilt to the object plane tilt therefore yieldsa smaller required image sensor size than when the image sensor tilt isfixed at zero, and eliminates perspective distortion from the capturedimage. Variable image sensor tilt is, however, a relatively costlyoption in practice, and also requires greater depth of field.

FIG. 63 illustrates the geometric relationship, for a given roll-relatedtilt θ of the pen's optical axis, between the pen's nib (point A), thepen's optical axis (CE), and the field of view window (FH). The tilt isagain defined to be clockwise positive from the vertical. With theexception of EQ 13, the preceding equations apply equally toroll-induced tilt. For roll-induced tilt the distance S from the nib tothe center of the window (i.e. |AC|) is zero rather than as defined byEQ 13.

For pitch-induced tilt, the magnitude of the tilt range is maximised bychoosing a minimum (negative) tilt and a maximum (positive) tilt whichhave the same image sensor requirement. Since, for pitch-induced tilt,the surface is more distant for negative tilt than for positive tilt ofthe same magnitude, the minimum has a smaller magnitude than themaximum. For roll-induced tilt they have the same magnitude.

As described above, the smallest features of the tag 4 are thestructures which encode the data bits, and these have a minimum diameterof 8 dots. This gives a maximum feature frequency f of about 7.9 per mmat 1600 dpi.

As calculated according to EQ 4 above, an equilateral triangular tilingof 256-dot diameter tags with no overlap between successive lines oftags requires a minimum field of view window diameter on the surface of598 dots, or about 9.5 mm at 1600 dpi.

Most people hold a pen at about +30° pitch and 0° roll. The inking ballof a ball-point nib loses effective contact with the surface beyondabout +50° pitch (i.e. 40° from the horizontal). A reasonable targetpitch range is therefore −10° to +50°, and a reasonable roll range 5-30°to +30°, bearing in mind greater limitations on combined pitch and rollas given by EQ 11.

The highly compact (1.5 mm²) Matsushita CCD image sensor (MatsushitaElectronic Corporation, and is described in a paper by Itakura, K TNobusada, N Okusenya, R Nagayoshi, and M Ozaki, “A 1 mm 50 k-Pixel ITCCD Image Sensor for Miniature Camera System”, IEEE Transactions onElectronic Devices, Volt 47, number 1, January 2000) is suitable for usein a compact device such as a pen. It has an available resolution of215×215 pixels. Assuming equal image and object space angular fields ofview, no image plane tilt, and a nib-to-window distance T of 4 mm,optimizing the geometry using EQ 16 and EQ 24 to achieve the desiredpitch and roll ranges stated above yields a pitch range of −16° to +48°(64°) and a roll range of −28° to +28° (56°) with a viewing distance Dof 30 mm and an angular field of view of 18.8° (α=9.4°). The availablepitch range is actually −21° to +43°, and this is mapped to close to thedesired range by pitching the optical axis at −5° relative to thephysical axis. Note that the tilt range can be expanded slightly byoptimizing a non-zero tilt of the image plane.

The overall pen tilt is thus confined to an elliptical cone whose majorangle is 64° in the pitch plane and whose minor angle is 56° in the rollplane.

The image sensing variables are summarized in Table 5.

TABLE 5 Image sensing variables variable Meaning α Object space field ofview half-angle α′ Image space field of view half-angle γ Pen yaw θObject plane tilt (i.e. overall pen tilt) θ′ Image plane tilt φ Penpitch ψ Pen roll ω Angular frequency in field of view D Normal viewingdistance d Tilted viewing distance f Surface feature frequency nSampling rate q Image sensor resolution S Distance from nib to center offield of view on surface (when θ = 0) T Distance from nib to edge offield of view on surface (when θ = 0)8.3 Tag Decoding8.3.1 Tag Image Processing and Decoding

Tag image processing is described earlier in Section 1.2.4. Itculminates in knowledge of the 2D perspective transform on the tag, aswell as the decoded tag data.

8.3.2 Inferring the Pen Transform

Once the 2D perspective transform is obtained which accounts for theperspective distortion of the tag in the captured image, as describedearlier, the corresponding discrete 3D tag transform with respect to thepen's optical axis can be inferred, as described below in Section 8.4.

Once the discrete 3D tag transform is known, the corresponding 3D pentransform can be inferred, i.e. the transform of the pen's physical axiswith respect to the surface. The pen's physical axis is the axis whichis embodied in the pen's shape and which is experienced by the pen'suser. It passes through the nib. The relationship between the physicalaxis and the optical axis is illustrated in FIG. 64.

It is convenient to define three coordinate spaces. In sensor space theoptical axis coincides with the z axis and the viewpoint is at theorigin. In pen space the physical axis coincides with the z axis and thenib is at the origin. In tag space the tag 4 lies in the x-y plane withits center at the origin. The tag transform transforms the tag 4 fromtag space to sensor space.

Sensor space is illustrated in FIG. 64. The labelling of points in FIG.64 is consistent with the labelling in FIG. 62. The viewpoint is at E,the sensed point is at G, and the nib is at A. The intersection point Gbetween the optical axis and the surface is referred to as the sensedpoint. In contrast with the geometry illustrated in FIG. 62 where thenib is considered as a point, here the nib is considered as a smallsphere. If the nib is curved, then the tilt of the physical axis affectsthe offset between the sensed point and the contact point between thenib and the surface. The center point K of the spherical nib, aboutwhich the physical axis pivots, is referred to as the pivot point.

The nib makes nominal contact with the surface at point A when theoptical axis is vertical. KA is defined to be parallel to the opticalaxis. When the pen is tilted, however, contact is at point L, as shownin FIG. 65. Given the radius R of the nib, the distance of the pivotpoint K from the surface, e.g. at A or L, is always R.

The discrete tag transform includes the translation of the tag centerfrom the sensed point, the 3D tag rotation, and the translation of thesensed point from the viewpoint.

Given the translation d of the sensed point from the viewpoint in thediscrete tag transform, and according to EQ 14, the sensed point isgiven by EQ 27.

Since the physical axis only differs from the optical axis by a ytranslation and x rotation (i.e. pitch), the physical axis lies in they-z plane. With reference to FIG. 64, where |AC|=S and |EC|=D (just asin FIG. 62), it is clear that in sensor space the position of the pivotpoint is given by EQ 28.

The vector from the sensed point to the pivot point is therefore givenby EQ 29.

The vector from the pivot point to the contact point is by definition asurface normal of length R. It is constructed by applying the 3D tagrotation M to a tag space surface normal, normalizing the result, andscaling by R, as shown in EQ 30 and EQ 31.

The vector from the sensed point to the contact point is then obtainedin accordance with EQ 32.

This is transformed into tag space by applying the inverse of the tagtransform 3D rotation, and is then added to the vector from the tagcenter to the sensed point, to yield the vector from the tag center tothe contact point in tag space, i.e. on the surface, in accordance withEQ 33.

This is finally added to the tag's absolute location, as implied by itstag ID, to yield the nib's desired absolute location in the taggedregion: see EQ 34.

The final step is to infer the pen's 3D orientation from the tag's 3Dorientation. The pen's discrete rotations are simply the inverses of thetag's discrete rotations, with the pen's pitch also including the effectof the pitch (φ_(sensor)) of the optical axis with respect to the pen'saxis, as defined in EQ 35, EQ 36 and EQ 37.

8.4 Inferring the Tag Transform

The image of the tag 4 captured by the image sensor contains perspectivedistortion due to the position and orientation of the image sensor withrespect to the tag. Once the perspective targets of the tag are found inimage space, an eight-degree-of-freedom perspective transform isinferred based on solving the well-understood equations relating thefour tag-space and image-space point pairs. The discrete transform stepswhich give rise to the image of the tag are concatenated symbolically,and a set of simultaneous non-linear equations is obtained by equatingcorresponding terms in the concatenated transform and the perspectivetransform. Solving these equations yields the discrete transform steps,which include the desired tag offset from the nib, 3D tag rotation, andviewpoint offset from the surface.

8.4.1 Modeling the Tag Transform

The transform of the tag 4 from tag space to image space can be modeledas a concatenation of the following transform steps:

-   -   x-y translate (by tag-to-viewpoint offset)    -   z rotate (by tag yaw)    -   x rotate (by tag pitch)    -   y rotate (by tag roll)    -   z translate (by tag-to-viewpoint offset)    -   perspective project (with specified focal length)    -   x-y scale (to viewport size)

These are concatenated symbolically to produce a single transform matrixwhich effects the tag transform. Table 7 summarizes the discretetransform variables used in the following sections, together with therange of each variable.

TABLE 7 Discrete transform variables and their ranges Vari- Unit ableAbbrev. Meaning transform Range γ — yaw 0 0 ≦ γ ≦ 2π φ — pitch 0 −π/2 <φ < π/2 ψ — roll 0 −π/4 < ψ < π/4 t_(x) A tag-to-viewpoint x offset 0 —t_(y) B tag-to-viewpoint y offset 0 — cosγ C cosine of yaw 1 −1 ≦ C ≦ 1sinγ D sine of yaw 0 −1 ≦ D ≦ 1 cosφ E cosine of pitch 1 0 < E ≦ 1 sinφF sine of pitch 0 −1 < F < 1 cosψ G cosine of roll 1 0 < G ≦ 1 sinψ Hsine of roll 0 −1 < H < 1 t_(z) I tag-to-viewpoint z offset — I < 0 1/λJ inverse focal length — J > 0 S — viewport scale — S > 0

Translate in x-y plane by t_(x) and t_(y) according to EQ 42 (whereA=t_(x) and B=t_(y)).

Rotate about z by γ according to EQ 43 (where C=cos(γ) and D=sin(γ)),giving EQ 44.

Rotate about x by φ according to EQ 45 (where E=cos(φ) and F=sin(φ)),giving EQ 46.

Rotate about y by ψ according to EQ 47 (where G=cos(ψ) and H=sin(ψ)),giving EQ 48, where K and L are defined by EQ 49 and EQ 50.

Translate in z by t_(z) according to EQ 51 (where I=t_(z)), giving EQ52.

Perspective project with focal length λ and projection plane at z=0according to EQ 53 (where J=1/λ), giving EQ 54.

Scale to viewport by S according to EQ 55, giving EQ 56.

Transform a point in the x-y plane (z=0) according to EQ 57, giving EQ58.

Finally, expand K and L, giving EQ 59.

8.4.2 2D Perspective Transform

Given an inferred eight-degree-of-freedom 2D perspective transformmatrix as defined in EQ 60, multiply by an unknown i to obtain thegeneral nine-degree-of-freedom form of the matrix, as shown in EQ 61.

Transform a 2D point according to EQ 62, giving EQ 63.

8.4.3 Inferring the Tag Transform

8.4.3.1 Equating Coefficients

Equating the coefficients in EQ 59 with the coefficients in EQ 63results in EQ 64 to EQ 72, being nine non-linear equations in 11unknowns.

These equations are augmented as required by the trigonometric identityrelating the sine and cosine of an angle (i.e. the sine and cosine ofany one of yaw, pitch and roll), as shown in EQ 73.

Given the sine and cosine of an angle, the corresponding angle isobtained using a two-argument arctan as shown in EQ 74.

8.4.3.2 Solving for X-Y Offset

EQ 66 can be simplified using EQ 64 and EQ 65 to give EQ 75 and then EQ76.

EQ 69 can be simplified using EQ 67 and EQ 68 to give EQ 77 and then EQ78.

EQ 72 can be simplified using EQ 70 and EQ 71 to give EQ 79 and then EQ80.

EQ 76 can be re-written as EQ 81, and EQ 78 can be re-written as EQ 82.Equating EQ 81 and EQ 82 and solving for B yields EQ 83 through EQ 85and finally EQ 86, which defines B.

Substituting the value for B into EQ 82 and simplifying yields EQ 87through EQ 90 and finally EQ 91, which defines A.

This therefore gives the x-y offset of the tag 4 from the viewpoint,since A=t_(x) and B=t_(y).

8.4.3.3 Solving for Pitch

From EQ 68, EQ 92 can be obtained.

From EQ 67, EQ 93 can be obtained.

From EQ 64, EQ 92 and EQ 93, EQ 94 can be obtained.

From EQ 65, EQ 92 and EQ 93, EQ 95 can be obtained.

From EQ 70, EQ 92 and EQ 93, EQ 96 can be obtained.

From EQ 71, EQ 92 and EQ 93, EQ 97 can be obtained.

From EQ 94, EQ 98 can be obtained.

From EQ 95, EQ 99 can be obtained.

From EQ 96, EQ 100 can be obtained.

From EQ 97, EQ 101 can be obtained.

From EQ 98 and EQ 99, EQ 102 and then EQ 103 can be obtained.

From EQ 100 and EQ 101, EQ 104 and then EQ 105 can be obtained.

From EQ 103 and EQ 105, EQ 106 and then EQ 107 can be obtained.

EQ 107 only has a valid basis if G and H are both non-zero. Since|ψ|<π/2, the cosine (G) of the roll is always positive and hencenon-zero. The sine of the roll (H) is only non-zero if the roll isnon-zero. Specific handling for zero pitch and roll is described inSection 6.7.3.10.

This therefore gives the magnitude of the sine of the pitch, sinceF=sin(φ), and hence the cosine (E) of the pitch by EQ 73, according toEQ 108.

Since |φ|<π/2, the cosine (E) of the pitch is always positive, so thereis no ambiguity when taking the square root. The sign of the sine (F),however, must be determined by other means, as described in Section6.7.3.9.

Given E and F, the pitch is then obtained, according to EQ 109.

8.4.3.4 Solving for Roll

From EQ 103, EQ 110 can be obtained.

From EQ 73, EQ 111 and then EQ 112 can be obtained.

This therefore gives the magnitude of the sine of the roll, sinceH=sin(ψ), and hence the cosine (G) of the roll by EQ 73, according to EQ113.

Since |ψ<π/4, the cosine (G) of the roll is always positive, so there isno ambiguity when taking the square root. The sign of the sine (H),however, must be determined by other means, as described in Section6.7.3.9.

Given G and H, the roll is then obtained according to EQ 114.

8.4.3.5 Solving for Yaw

From EQ 73, EQ 92 and EQ 93, EQ 115 and then EQ 116 can be obtained.

From EQ 92 and EQ 116, EQ 117 and then EQ 118 can be obtained.

From EQ 92 and EQ 116, EQ 119 and then EQ 120 can be obtained.

In EQ 116, and hence EQ 118 and EQ 120, the sign of the square root isdetermined by the sign of i, which can be determined from EQ 80, givingEQ 121.

Since I (t_(z)) is negative, J (1/λ) is positive, and IJ<−1 (because|t_(z)|>λ), then EQ 122 holds.

Given C and D, the yaw is then obtained according to EQ 123.

8.4.3.6 Solving for Viewport Scale

The cosine (C) and sine (D) of the yaw are by definition neversimultaneously zero. Since the cosine (E) of the pitch is never zero,either EQ 67 or EQ 68 can therefore always be used to determine theviewport scale (S).

If D is non-zero, then from EQ 67, EQ 124 can be obtained.

Otherwise, if C is non-zero, then from EQ 68, EQ 125 can be obtained.

8.4.3.7 Solving for Focal Length

Similarly, since the cosine (G) of the roll is never zero, either EQ 70or EQ 71 can be used to determine the inverse focal length (J), so longas either the pitch or roll is non-zero. However, the signs of the sines(F and H) of the pitch and roll may not be known. However, the sign ofthe product (FH) of the sines of the pitch and roll is given by EQ 103,as shown in EQ 126.

The sign can be assigned arbitrarily to F, since the sign of J is knowna priori. If gi is non-zero, then from EQ 70, EQ 127 can be obtained.

If hi is non-zero, then from EQ 71, EQ 128 can be obtained.

In practice, the choice between using EQ 127 and EQ 128 is based onwhich of gi and hi has the larger magnitude. The inverse focal length isunknown if gi and hi are both zero, i.e. if the pitch and roll are bothzero.

8.4.3.8 Solving for Z Offset

Once the inverse focal length (J) is known, the z offset (I) is obtainedfrom EQ 80, according to EQ 129.

Again, the z offset (I) is unknown if the inverse focal length (J) isunknown, i.e. if the pitch and roll are both zero.

8.4.3.9 Determining Direction of Pitch and Roll

The sign of the product (FH) of the sines of the pitch and roll is givenby EQ 126. Since −π/4<ψ<π/4, a roll adjustment of +π/4 can be introducedto ensure the roll is always positive, without invalidating any otherassumptions. Once the roll adjustment is introduced, EQ 126 gives thesign of the sine (F) of the pitch alone.

The roll adjustment is introduced as follows. The viewport scale (S),inverse focal length (J), and z offset (I) are all computed asdescribed. A 3D transform matrix is created from the 2D perspectivetransform matrix. The inverses of the viewport scale, focal lengthprojection and z translation are applied to the 3D matrix in reverseorder. The roll adjustment is then applied by pre-multiplying the matrixby a π/4 y rotation matrix. The roll, pitch and yaw are computed asdescribed. Since the roll is positive, the pitch direction is now known.The π/4 roll adjustment is finally subtracted from the roll to give theactual roll.

When the roll and pitch are both zero, the focal length and z offset areboth unknown as described above. However, in this case there is no needto adjust the roll since the pitch and roll are already known.

8.4.3.10 Handling Zero Pitch and Roll

When either the pitch or roll is zero, the general solution based on EQ107 becomes invalid. The table of FIG. 85 shows the 12 degenerate formsof EQ 64 through EQ 71 which result when the yaw is variously zero (orπ), π/2 (or 3π/2), and non-zero, and the pitch and roll are variouslyzero and non-zero. The table of FIGS. 86 and 87 sets out the requiredlogic for detecting and handling cases where the pitch and/or roll arezero, with each case motivated by zeros appearing in the table of FIG.85. The cases in the table of FIG. 85 are labelled with the case numbersfrom the table of FIGS. 86 and 87.

CONCLUSION

The present invention has been described with reference to a preferredembodiment and number of specific alternative embodiments. However, itwill be appreciated by those skilled in the relevant fields that anumber of other embodiments, differing from those specificallydescribed, will also fall within the spirit and scope of the presentinvention. Accordingly, it will be understood that the invention is notintended to be limited to the specific embodiments described in thepresent specification, including documents incorporated bycross-reference as appropriate. The scope of the invention is onlylimited by the attached claims.

1. A system for performing computer-based operations, the systemcomprising: a computer system configured for: receiving a print requestfor printing a document; allocating and retrieving a unique documentidentifier for said document upon receipt of said print request; andassociating the document identifier with a description of the document,the description associating at least one instruction with one or morepositions in the document; a printer for printing the documentcontaining visible information and coded data tags on a surfacesimultaneously, each coded data tag encoding position on the surface andeach coded data tag further identifying the document identifier; and anoptically imaging pen for sensing at least one coded data tag duringhandwritten input on the document and for generating a signal indicativeof the sensed coded data tag; wherein the computer system is furtherconfigured for: receiving the signal from the pen; determining thedocument identifier and the position; retrieving the descriptionassociated with the document identifier; and relating the position tothe at least one instruction using the retrieved description andexecuting the at least one instruction.
 2. A system as claimed in claim1, in which the printer is networked to the computer system, wherein thesensor sends the signal to the printer which communicates the signal tothe computer system.
 3. A system as claimed in claim 1, wherein thecoded data tags are printed using infrared-absorptive ink.
 4. A systemas claimed in claim 1, wherein the printer prints the coded data tags tofill the surface.
 5. A system as claimed in claim 1 wherein, whensignals indicative of a sequence of sensed coded data tags are received,the computer system is further configured to identify a path traversedby the sensor over the coded data tags.
 6. The system as claimed inclaim 1, wherein the at least one instruction is an instruction toupdate the description with information corresponding to the handwritteninput.